How to reduce TTFT in production: practical patterns, implementation strategies, and edge cases to watch for.

TL;DR: Prefix caching can deliver 70-90% TTFT reduction for cached requests with stable prompts (first request still incurs full prefill cost). Tiered context loading reduces token count by 30-70% depending on query distribution, translating to proportional TTFT improvements. Batch processing requires careful monitoring of batch_size × prompt_tokens to avoid out OutOfMemory (OOM) errors. This post covers practical patterns, provider-specific considerations, and common production edge cases.

Context from Part 1

In Part 1, I covered how prompt size directly impacts TTFT through the prefill stage, where the model generates its KV cache. The relationship is linear: approximately 0.20-0.24ms per token, though this varies by provider and infrastructure.

This post will cover a few practical strategies to reduce these delays.

Mitigation and Optimisation

Several techniques can be used to manage the delays associated with large prompts. These include:

  • Prefix Caching
  • Parallel Processing (Cake)
  • Prompt Engineering

Prefix Caching

Prefix caching allows the system to store and reuse the KV cache for frequently used or repeated prompt prefixes [#2].

This is particularly effective for:

  • System instructions that remain constant across conversations
  • Shared documents or context that appear in multiple requests
  • Few-shot examples that are reused

How it works:

  1. The first request processes the full prompt and generates the KV cache
  2. The system saves the KV cache for the prefix portion
  3. Subsequent requests with the same prefix skip recomputation
  4. Only the new tokens (user query) need to be processed

Impact:

First request (3,000 token prompt):
Prefill: 600ms

Subsequent requests (2,500 token cached prefix + 500 new tokens):
Prefill: 100ms (83% reduction)

👉 Takeaway: If the system instructions or context are stable, prefix caching can significantly reduce TTFT for subsequent requests. Provider implementations vary widely (see Provider-Specific Variations below).

Parallel Processing (Cake)

New systems like “Cake” attempt to reduce TTFT by parallelizing KV cache generation [#3].

How it works:

  • Simultaneously compute the cache on the GPU from the start of the prompt
  • Load a saved cache from disk in reverse order
  • Merge the two caches as they meet in the middle

This reduces TTFT by overlapping computation and I/O, particularly effective when:

  • The prefix is cached on disk but needs to be loaded
  • The suffix is new and needs to be computed
  • Disk I/O and GPU computation can run in parallel

Prompt Engineering

Splitting complex prompts into parallel smaller prompts reduces perceived delay by pre-processing context before user queries arrive [#1].

This approach is particularly effective for:

  • Document-heavy applications where users upload files before querying
  • Long-running conversations where history can be pre-summarized
  • Multi-tenant systems where context is shared across users

For implementation details, see Pattern 4: Background Context Pre-processing.

The Hidden Costs

Often overlooked considerations:

  1. Predictive loading logic complexity:
  • Which documents will users query? Wrong predictions waste compute.
  • When to invalidate pre-processed summaries?
    • Too aggressive → cache misses
    • Too conservative → stale data
  • How to prioritize pre-processing under resource constraints?
  1. Background compute economics:
  • Background processing increases costs in low-utilization scenarios
  • 🚨️ Break-even occurs when queries significantly outnumber uploads (say, 3:1 or higher)
  • Below that threshold, pre-processing costs may exceed the latency benefits users experience
  1. Cache invalidation complexity:
  • Document updates require re-processing all dependent summaries
  • In collaborative environments, updates may occur faster than summary generation

Trade-off Decision Framework

This pattern delivers maximum ROI when:

  • Context change frequency is LOW relative to query frequency (documents remain stable for hours/days, not minutes)
  • Query patterns are predictable (e.g., FAQ scenarios)

Skip this pattern when:

  • Documents change frequently (collaborative editing, real-time feeds)
  • Query patterns are unpredictable (exploratory analysis)
  • Team lacks infrastructure to monitor cache hit rates

Practical Implementation Patterns

Here are a few effective patterns that can be used in production systems:

Pattern 1: Tiered Context Loading

The context is loaded progressively based on query complexity.

def get_context(user_query, complexity_threshold=0.7):
    # Always load: lightweight system prompt (~200 tokens)
    context = get_system_prompt()
    
    # Conditional: add history only if needed
    if needs_history(user_query):
        context += get_recent_history(max_tokens=500)
    
    # Conditional: add documents only for complex queries
    if query_complexity(user_query) > complexity_threshold:
        context += get_relevant_docs(max_tokens=1000)
    
    return context

# NOTE: The pseudo-code is only for illustration

How token reduction translates to TTFT improvement:

Query complexity classification determines context size:

  • Simple queries (e.g., “What is X?”) → 200 tokens (system prompt only)
  • Medium queries (e.g., “How does X work?”) → 700 tokens (+ recent history)
  • Complex queries (e.g., “Compare X and Y in context Z”) → 1,700 tokens (+ documents)

Token reduction analysis:

Assuming typical query distribution: 40% simple / 30% medium / 30% complex

Average tokens with tiered loading:
(0.4 × 200) + (0.3 × 700) + (0.3 × 1,700) = 800 tokens

Without tiering (always full context): 1,700 tokens

👉 Reduction: (1,700 - 800) / 1,700 = 53%

This translates to proportional TTFT improvement:

  • Original TTFT: 1,700 × 0.24ms = 408ms
  • Optimised TTFT: 800 × 0.24ms = 192ms
  • Improvement: 216ms (53% reduction)

The 30-70% range depends on query distribution:

  • FAQ-heavy applications (60% simple): ~65% reduction
  • Analysis-heavy applications (60% complex): ~35% reduction

Implementation note: Profile your query distribution for 1-2 weeks to understand potential impact before starting implementation.

💼 Recent Experience: In recent projects I have worked on, query distribution variance is significant:

  • Knowledge base systems: 70% simple queries
  • Document analysis tools: 60% complex queries

References:

  • OpenViking implements a similar 3-tier approach (L0/L1/L2) [#7]
  • Context management tools commonly trigger tiering at >70% context utilization [#8]
  • Architecture patterns for tiered context loading [#9]

Pattern 2: Aggressive Summarisation

Aggressively summarise conversation history.

def prepare_conversation_context(messages, max_tokens=1000):
    recent_messages = messages[-5:]  # Keep last 5 verbatim
    older_messages = messages[:-5]
    
    if len(older_messages) > 0:
        # Summarise older context
        summary = summarise(older_messages, target_tokens=200)
        return summary + recent_messages
    
    return recent_messages

# NOTE: The pseudo-code is only for illustration

Trade-off:

  • Token reduction: 2,000 → 1,000 (50%)
  • TTFT improvement calculation:
    • Original: 2,000 × 0.20ms = 400ms (lower) to 2,000 × 0.24ms = 480ms (upper)
    • Optimised: 1,000 × 0.20ms = 200ms (lower) to 1,000 × 0.24ms = 240ms (upper)
    • Improvement: 200-240ms reduction
  • Context loss: Minimal (recent messages preserved)

🚨️ Monitor summary quality. Aggressive summarization can lose:

  • Nuanced context
  • User preferences mentioned earlier
  • Important constraints from prior messages

Pattern 3: Cached System Instructions

Use prefix caching for stable system prompts.

# Mark prefix for caching (implementation varies by provider)
system_prompt = """
You are a helpful assistant...
[1,000 tokens of instructions]
"""

# First call: full prefill
response = llm.complete(
    prompt=system_prompt + user_query,
    cache_prefix=True  # Provider-specific
)

# Subsequent calls: cached prefix
# Only user_query tokens are processed
response = llm.complete(
    prompt=system_prompt + user_query,
    cache_prefix=True
)

# NOTE: The pseudo-code is only for illustration

Impact:

  • First call: 1,100 tokens → TTFT calculation:
    • Lower bound: 1,100 × 0.20ms = 220ms
    • Upper bound: 1,100 × 0.24ms = 264ms
    • TTFT: 220-264ms
  • Cached calls: 100 tokens → TTFT calculation:
    • Lower bound: 100 × 0.20ms = 20ms
    • Upper bound: 100 × 0.24ms = 24ms
    • TTFT: 20-24ms
    • Reduction: 90% improvement (from 220-264ms to 20-24ms)

Pattern 4: Background Context Pre-processing

❌ Synchronous (naive):
User query triggers → Process 3,600 tokens → TTFT: 720-864ms

✅ Asynchronous (optimised):
Background: Pre-process 2,500 tokens (docs + history)
User query triggers → Process 1,000 tokens → TTFT: 200-240ms
Improvement: 72-77% TTFT reduction

When this pattern matters:

  • Document-heavy applications where users upload files before querying
  • Long-running conversations where history can be pre-summarized
  • Multi-tenant systems where context is shared across users

For implementation considerations, see The Hidden Costs and Trade-off Decision Framework.

Things To Take Note Of

1. TTFT ≠ Generation Speed

TTFT measures how long until the first token appears.

TTFT is not related to tokens-per-second during generation.

TTFT measures: How long until the first token appears (prefill phase) Generation speed measures: How many tokens per second after generation starts (decode phase)

Prefill Phase (TTFT):
├─ Process all input tokens in parallel
├─ Generate KV cache for entire input
└─ Computational cost: Linear with input tokens

Decode Phase (Generation Speed):
├─ Process one token at a time
├─ Use cached KV values from prefill
└─ Computational cost: Linear with output tokens

Two kinds of scenarios:

  • Fast TTFT + slow generation (small prompt, slow model)
  • Slow TTFT + fast generation (large prompt, fast model)

Optimise both independently.

Metric Optimisation Target Techniques
TTFT Reduce input tokens Prefix caching, prompt compression, tiered context loading
Generation speed Increase throughput Quantization, speculative decoding, batch processing

2. Batch Size Amplifies KV Cache Costs

The KV cache scales linearly with both the number of tokens and the batch size. This multiplication can quickly overwhelm GPU memory.

Memory calculation:

From NVIDIA’s formula:

Total KV cache size (bytes) = batch_size
 × sequence_length × 2
 × num_layers
 × hidden_size
 × sizeof(FP16)

For a 7B parameter model like Llama 2 (model card):

  • Layers: 32
  • Hidden size: 4,096
  • Precision: FP16 (2 bytes per value)

Example 1: Single request:

Prompt: 2,000 tokens
Batch size: 1

Using NVIDIA's formula:
KV cache = batch_size × sequence_length × 2 × num_layers × hidden_size × sizeof(FP16)
         = 1 × 2,000 × 2 × 32 × 4,096 × 2 bytes
         = 1,048,576,000 bytes
         ≈ 1 GB

Example 2: Batch of 8 requests:

Prompt: 2,000 tokens each
Batch size: 8

Using NVIDIA's formula:
KV cache = batch_size × sequence_length × 2 × num_layers × hidden_size × sizeof(FP16)
         = 8 × 2,000 × 2 × 32 × 4,096 × 2 bytes
         = 8,388,608,000 bytes
         ≈ 8 GB

Why this matters:

A GPU with 24GB VRAM might handle:

  • 10-12 concurrent requests with 2,000-token prompts
  • Only 4-5 concurrent requests with 5,000-token prompts

💡 Takeaway: In production batch inference, monitor batch_size × prompt_tokens, not just batch size alone. Naive batch processing will hit OOM sooner or later - consider adaptive batch sizing.

3. Provider-Specific Variations

Different LLM providers implement prefill and caching differently, leading to significant TTFT variations for the same prompt.

  • Anthropic Claude: Uses explicit prompt caching that requires developers to mark content with cache_control parameters [#4]. Enables latency reductions up to 85% for long prompts when caching is enabled [#5]. Cache entries have a 5-minute TTL (default) or 1-hour TTL (optional, higher cost).

  • OpenAI: Implements automatic prompt caching enabled by default for prompts exceeding 1,024 tokens, with 50% cost reduction for cached tokens [#6].

    • Caching occurs transparently without code changes.
    • Setting the prompt_cache_key parameter helps improve cache hit rates as a result of cache routing (i.e. requests are routed to a machine based on a hash of the initial prefix of the prompt, thus higher chance of a cache hit).
    • Cache lifetime can be extended using the prompt_cache_retention parameter.
{
  "model": "gpt-5.1",
  "input": "Your prompt goes here...",
  "prompt_cache_retention": "24h"
}
# From https://platform.openai.com/docs/guides/prompt-caching

👉 Always benchmark TTFT with your specific provider and workload patterns rather than relying on general estimates.

4. Edge Cases

Common edge cases likely to occur in multi-user production environment.

  1. Power users & long conversation history:

    Regular user: 5 messages, 200 tokens
Power user: 50 messages, 5,000 tokens

Outcome: Power user waits ~25× longer (when no optimisation inplace)

  2. Large document uploads:

    Typical query: 300 tokens
With 100-page PDF: 50,000+ tokens

Outcome: TTFT increase, 60ms → 10,000ms (167× slower)

  3. Multi-turn debugging sessions:

    Turn 1: 500 tokens
Turn 5: 2,500 tokens (accumulated context)
Turn 10: 5,000 tokens
Turn 20: 10,000 tokens

Each turn gets progressively slower without context management

How To Mitigate Edge Cases?

Mitigation Strategy 1: Progressive Degradation

def prepare_context(user_query, conversation_history):
    token_budget = 3000  # Target token limit
    
    # Essential: Always include
    context = get_system_prompt()  # 500 tokens
    context += user_query  # ~100 tokens
    
    remaining_budget = token_budget - count_tokens(context)
    
    # Optional: Add based on remaining budget
    if remaining_budget > 1000:
        # Include recent history
        recent = get_recent_messages(
            conversation_history, 
            max_tokens=min(remaining_budget, 2000)
        )
        context += recent
        remaining_budget -= count_tokens(recent)
    
    if remaining_budget > 500:
        # Include relevant documents
        docs = get_relevant_docs(
            user_query,
            max_tokens=remaining_budget
        )
        context += docs
    
    return context

# NOTE: The pseudo-code is only for illustration

Mitigation Strategy 2: Aggressive Summarisation At Thresholds

def manage_conversation_context(messages):
    total_tokens = sum(count_tokens(m) for m in messages)
    
    # Threshold-based compression
    if total_tokens < 2000:
        return messages  # No compression needed
    
    elif total_tokens < 5000:
        # Summarise older messages, keep recent verbatim
        old_messages = messages[:-10]
        recent_messages = messages[-10:]
        
        summary = summarise(old_messages, target_tokens=500)
        return [summary] + recent_messages
    
    else:  # > 5000 tokens - aggressive compression
        # Keep only last 5 messages + summary of all prior
        old_messages = messages[:-5]
        recent_messages = messages[-5:]
        
        summary = summarise(old_messages, target_tokens=300)
        return [summary] + recent_messages

# NOTE: The pseudo-code is only for illustration

Implementation Strategy: A Phased Approach

The prioritisation for the optimisations would be based on ROI and implementation effort:

Phase 1: Foundation

1. TTFT Monitoring

Why first? One cannot optimise what one does not measure. 🚨️ Teams who skip this stand to risk waste cycles on premature optimisation.

  • Percentile tracking (P50, P95, P99), per endpoint
  • User segment analysis (power users vs typical)
  • Minimal effort required for implementation: <1 week, assuming an engineer with 2-3 years of experience
  • Additional cost: minimal (since only additional logging involved)
  • Impact: (supports a significant impact)

2. Prefix Caching for System Prompts

Why second? Highest ROI with lowest implementation risk

  • Implement provider-native caching
  • Monitor cache hit rates
  • Target stable prompts which have high hit rate (> 70%)
  • Minimal effort required, since minimal code changes
  • Impact: significant

Phase 2: Conditional Optimisations

3. Tiered Context Loading

Why third: Adds complexity (query classification), so only implement if Phase 1 monitoring shows high variance in prompt sizes.

  • Build a query classifier (determines if query is simple/medium/complex)†
  • Implement conditional context loading
  • Monitor metrics:
    • Classification accuracy (target: >85%)
    • TTFT improvement per tier
    • Response quality vs full context baseline
  • Effort required: 1-2 weeks for 2 engineers
  • Impact: 30-50% additional TTFT reduction

† Query Classifier Implementation: Options include fine-tuned BERT (best accuracy), GPT-4o-mini (fastest deployment), or rule-based heuristics (simplest). Input: raw query text. Output: simple/medium/complex + confidence. Example: “What is X?” → simple (system prompt only), “Compare X and Y considering Z” → complex (full context).

4. Aggressive Summarisation

  • Trigger implementation if conversation history consistently ranks beyond the configured percentile threshold
  • Prerequisites:
    • Requires a summary quality evaluation pipeline to be in place prior to development
    • Human-in-the-loop validation
    • Rollback mechanisms

Why last: Introduces non-determinism and requires quality monitoring infrastructure

Advanced & Emerging Techniques

Automatic Prefix Caching in vLLM (Self-Hosted)

For self-hosted deployments, vLLM’s Automatic Prefix Caching detects and reuses shared prefixes across requests without manual intervention. It works transparently with paged attention, making it ideal for multi-user or multi-turn workloads.

Key benefits:

  • No code changes needed beyond standard vLLM server launch
  • Excellent cache hit rates when many requests share system prompts or RAG context
  • Combines well with paged attention for memory efficiency under variable batch sizes

Usage example (OpenAI-compatible server):

# Automatic prefix caching is enabled by default in recent vLLM versions
python -m vllm.entrypoints.openai.api_server \
    --model meta-llama/Meta-Llama-3.1-70B-Instruct \
    --tensor-parallel-size 8 \
    --max-model-len 32768 \
    --enable-prefix-caching  # Optional flag (often default-on)

Subsequent requests with identical prefixes automatically reuse the KV cache. Monitor cache hits via vLLM metrics (e.g., vllm:prefix_cache_hit_rate).

Prefill-Decode Disaggregation

Emerging in advanced serving systems (e.g., DistServe, Moreh variants), this runs prefill on separate high-memory nodes while decode uses optimised throughput nodes, reducing TTFT bottlenecks in mixed workloads.

vLLM supports an experimental disaggregated prefill mode via separate prefill/decode instances and KV transfer connectors.

Example configuration (simplified NixlConnector):

# Prefill instance
python -m vllm.entrypoints.openai.api_server \
    --kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_producer"}'

# Decode instance
python -m vllm.entrypoints.openai.api_server \
    --kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_consumer"}'

NOTE:

  • This is only available in self-hosted/open-source engines like vLLM.
  • Major hosted providers (such as Anthropic, OpenAI, Azure OpenAI, etc.) do not expose disaggregation controls via their APIs - prefill and decode are handled internally on shared infrastructure.

Chunked or Streaming Prefill

Some engines support generating early tokens before full prefill completes (speculative/partial/chunked prefill), overlapping phases for lower perceived TTFT. TensorRT-LLM and certain research systems implement this effectively.

However, this remains experimental and model-dependent. Major hosted providers (such as Anthropic, OpenAI, Azure OpenAI, etc.) do not support true chunked/streaming prefill via their APIs. Streaming only begins after complete prefill. Early token generation would require custom self-hosted setups.

ROI Calculation

Daily savings = V requests/day × (X-Y) ms improvement × $C per ms
Break-even days = Engineering cost / Daily savings

Where:

  • V = Daily request volume
  • X = Current P95 TTFT (ms)
  • Y = Target P95 TTFT (ms)
  • $C = Business cost per millisecond of latency
  • Engineering cost = Z days × engineer daily rate

An Example,

Given:

  • Current P95 TTFT: 2,400ms
  • Target P95 TTFT: 1,200ms (improvement: 1,200ms)
  • Daily request volume: 200 requests/day
  • Business cost: $6,000/hour = $1.67/second = $0.00167/ms
  • Engineering effort: 5 days @ $1,000/day = $5,000

Calculate:

  • Daily savings = 200 × 1,200ms × $0.00167/ms = $400.80/day
  • Break-even = $5,000 / $400.80 = 12.5 days

Conclusion:

  • The optimisation pays for itself in 12.5 working days.
  • After that, it delivers $400.80/day in ongoing savings (~$100K annually).

💡 Key insight: At 200 requests/day with 1,200ms improvement, even a 5-day engineering effort breaks even in under 3 weeks.

References

  1. How input token count impacts the latency of AI chat tools (Jan, 2026)
  2. Prefix caching
  3. Compute or Load KV Cache? Why not both? (arXiv, Oct, 2024)
  4. Prompt caching with Claude
  5. Prompt caching announcement
  6. OpenAI Prompt Caching Documentation
  7. OpenViking Tiered Context Loading
  8. MCP Context Loader
  9. DeepWiki Context Engineer Architecture