How to Lower Transcription Latency in Voice AI Systems: Practical Tips

TL;DR

Most voice AI systems hit 200-800ms transcription latency because they batch audio chunks instead of streaming. VAPI's streaming STT with partial transcripts cuts this to 80-150ms. Use Twilio's WebSocket connection for raw PCM audio (not compressed), enable early partial results, and implement barge-in detection on interim transcripts—not finals. This cuts time-to-first-token by 60% and prevents awkward silence gaps in real-time conversations.

Prerequisites

API Keys & Credentials

VAPI API key (generate at dashboard.vapi.ai)

Twilio Account SID and Auth Token (from console.twilio.com)

OpenAI API key for LLM inference (gpt-4 or gpt-4-turbo recommended for sub-200ms response times)

System Requirements

Node.js 18+ (async/await support required for streaming handlers)

Minimum 2GB RAM for session state management (production: 8GB+ for 100+ concurrent calls)

Network: <50ms latency to VAPI and Twilio endpoints (use regional…