glossary
verified July 2026
Time to first audio (TTFA)
Time to first audio is the delay between handing text to a TTS system and receiving the first byte of playable audio. For a voice agent it is the number that decides whether the agent answers or hesitates.
01
Where the milliseconds land
The scale below is one second — the rough ceiling of a caller’s patience. Everything about a TTFA claim is where it sits on this line, and how far its p95 drifts from its p50.
Fig. — One second of caller patience, to scale
107 / 108 ms
p50 / p95, single stream, production API
200 ms
human turn-taking rhythm
1,000 ms
callers start interrupting
107 / 108 ms
p50 / p95, single stream, production API
200 ms
human turn-taking rhythm
1,000 ms
callers start interrupting
The gap between p50 and p95 is the honesty of a latency claim — ours is one millisecond on a single stream, which is why the two marks sit on top of each other.
02
TTFA vs model latency vs TTFB
Model latency counts only the neural network’s forward pass — no API gateway, no queue, no network. Time to first byte (TTFB) is a web metric that counts any response byte, including headers. TTFA counts what the caller experiences: request sent, first audible sample back. A vendor quoting model-only latency can be honest and still describe a number you will never see in production.
03
How to measure it honestly
- Measure server time on the production endpoint, not a staging box or a co-located benchmark.
- Use call-shaped load — short conversational utterances, not paragraph batches.
- Report percentile pairs: a p50 without a p95 hides the tail your callers live in.
- State what the figure includes. Ours: 107 ms p50, 108 ms p95, server time to first audio on the production API.
See also
Related sheets.
benchmark
5 rules
How we measure TTS latency
The methodology behind every figure on this site: server-time TTFA on the production API under call-shaped load, percentile pairs, published conditions.
glossary
~100 ms
Streaming text-to-speech
Streaming TTS returns audio in chunks as synthesis proceeds instead of one file at the end. Why voice agents require it, and what to check in a streaming API.
glossary
< 1
Real-time factor (RTF)
RTF is synthesis time divided by audio duration. Why RTF below 1 is table stakes, and why time to first audio matters more for conversations.
Every term on this page is measurable on a live call — watch the readout while your own script synthesizes.
See the measured figures live