Making Turborepo 96% faster with agents, sandboxes, and humans

Turborepo 2.9 achieves up to 96% faster task graph computation through eight days of iterative optimization. The engineer combined unattended AI coding agents, human-in-the-loop review, and Vercel Sandboxes for clean benchmarking. Key wins came from parallelizing sequential operations (git index, filesystem walks, lockfile parsing), eliminating redundant heap allocations, reducing syscalls by replacing git subprocesses with gix-index, and removing legacy .tar fallback code from Turborepo's Golang era. A critical insight was converting Chrome Trace JSON profiles to Markdown format, which dramatically improved AI agent suggestion quality. Time to First Task on a 1,000+ package monorepo dropped from 8.1 seconds to 716 milliseconds.