# Glossary — AI API latency terms
Updated July 24, 2026.

- **Time to first byte (TTFB)** — The time from starting a request to receiving the first byte of the response, including DNS, TCP and TLS setup. Here it is the primary edge-latency metric and isolates network and endpoint speed from model compute.
- **Time to first token (TTFT)** — For a streamed AI completion, the time from sending the request to the first token arriving. It captures queueing and model start-up, not just the network.
- **p50 (median)** — The middle value: half of measured requests were faster and half slower. Represents a typical request.
- **p95** — The 95th percentile: only 5% of requests were slower. Captures the slow tail that users notice, which averages hide.
- **Uptime** — The share of probes that returned a successful response over the measurement window, expressed as a percentage.
- **Edge latency** — Latency attributable to the network path and the provider's connection termination (DNS/TCP/TLS/first byte), as opposed to model inference time.
- **Cold start** — Extra latency when an endpoint or model must be initialised because it was idle, versus a warm path already serving traffic.
- **Region** — The geographic location a measurement was taken from. Latency depends on distance to the provider's infrastructure, so rankings differ by region.
- **Speed Index** — A composite 0-100 score comparing providers across regions: in each region, fastest_p50 / provider_p50 x 100 (100 = as fast as the region leader), averaged over measured regions. Derived from measured p50, not a subjective rating.

Source: https://llmlatency.dev/glossary