Story

I built a voice agent from scratch that averages ~400ms end-to-end latency (phone stop → first syllable). That’s with full STT → LLM → TTS in the loop, clean barge-ins, and no precomputed responses.

What moved the needle:

Voice is a turn-taking problem, not a transcription problem. VAD alone fails; you need semantic end-of-turn detection.

The system reduces to one loop: speaking vs listening. The two transitions - cancel instantly on barge-in, respond instantly on end-of-turn - define the experience.

STT → LLM → TTS must stream. Sequential pipelines are dead on arrival for natural conversation.

TTFT dominates everything. In voice, the first token is the critical path. Groq’s ~80ms TTFT was the single biggest win.

Geography matters more than prompts. Colocate everything or you lose before you start.