Latency Overhead Troubleshooting

Use this guide when you see unexpected latency overhead between LiteLLM proxy and the LLM provider.

Quick Checklist

1. Collect the x-litellm-overhead-duration-ms response header — this tells you LiteLLM's total overhead on every request. Start here.
2. Is DEBUG logging enabled? This is the #1 cause of latency with large payloads.
3. Are you sending large base64 payloads? (images, PDFs) — see Large Payload Overhead.
4. Enable detailed timing headers to pinpoint where time is spent.

Diagnostic Headers

x-litellm-overhead-duration-ms (always on)

Every response from LiteLLM includes this header. It shows the total latency overhead in milliseconds added by LiteLLM proxy (i.e. total response time minus the LLM API call time). Collect this on every request to understand your baseline overhead.

curl -s -D - http://localhost:4000/v1/chat/completions \
  -H "Authorization: Bearer sk-..." \
  -d '{"model": "gpt-4o", "messages": [{"role": "user", "content": "hi"}]}' \
  2>&1 | grep x-litellm-overhead-duration-ms

x-litellm-callback-duration-ms (always on)

Shows time spent building callback/logging payloads (ms). If this is high (>100ms), your payloads may be too large for efficient logging.

curl -s -D - http://localhost:4000/v1/chat/completions \
  -H "Authorization: Bearer sk-..." \
  -d '{"model": "gpt-4o", "messages": [{"role": "user", "content": "hi"}]}' \
  2>&1 | grep x-litellm

Detailed Timing Breakdown (opt-in)

Set LITELLM_DETAILED_TIMING=true to get per-phase timing in response headers:

Header	What it measures
x-litellm-timing-pre-processing-ms	Auth, routing, request processing (before LLM call)
x-litellm-timing-llm-api-ms	Actual LLM API call duration
x-litellm-timing-post-processing-ms	Response processing (after LLM returns)
x-litellm-timing-message-copy-ms	Message copy time in logging layer

# Enable detailed timing
export LITELLM_DETAILED_TIMING=true

Large Payload Overhead

When sending large payloads (>1MB, e.g. base64-encoded images/PDFs), three things can add overhead:

1. DEBUG Logging (most common)

When LITELLM_LOG=DEBUG or set_verbose=True is enabled, every request payload is serialized with json.dumps(indent=4) synchronously. For a 2MB+ payload, this alone can take 2-5 seconds.

Fix: Don't use DEBUG logging in production. Use INFO level instead:

export LITELLM_LOG=INFO

If you need DEBUG logging but have large payloads, you can increase the size threshold for full payload logging:

# Only fully serialize payloads under 100KB for DEBUG logs (default)
export MAX_PAYLOAD_SIZE_FOR_DEBUG_LOG=102400

2. Base64 in Logging Payloads

Callback payloads (sent to Langfuse, etc.) include message content. Large base64 strings are automatically truncated to size placeholders in logging payloads.

You can control the truncation threshold:

# Max base64 characters before truncation (default: 64)
export MAX_BASE64_LENGTH_FOR_LOGGING=64

Environment Variables Reference

Variable	Default	Description
LITELLM_DETAILED_TIMING	false	Enable per-phase timing headers
MAX_PAYLOAD_SIZE_FOR_DEBUG_LOG	102400	Max payload bytes for full DEBUG serialization
MAX_BASE64_LENGTH_FOR_LOGGING	64	Max base64 chars before truncation in logging