Your codex exec is wasting ~900 ms per turn. I measured where.

I spent a weekend rebuilding the JavaScript harness of OpenAI's Codex CLI on Bun. I expected a runtime story. I got an architecture story — and one config line you can use today without touching my fork.

The 30-second version

Codex is a Rust engine with a thin JavaScript layer on top: an npm launcher, a TypeScript SDK, and a pnpm/jest/tsup toolchain. I swapped that layer to Bun, added a client that keeps one codex app-server process alive instead of spawning a fresh process per turn, and measured everything with hyperfine and mitata, p50 and p99.

Ten short turns, same Rust engine, wall clock on my machine:

10 turns total telemetry on telemetry off