Your Node.js app is slow to start. You just don't know which module to blame.

Last month I was debugging a startup regression at work. Our Node.js service went from ~300ms boot to nearly 900ms overnight. No new features. No infra changes. Just a routine dependency bump. The usual approach? Comment out requires one by one. Bisect package.json. Stare at --cpu-prof output and pretend to understand V8 internals. I wanted something simpler: run one command, see which module is eating my startup time, and know if the cost is in the module itself or in everything it drags in. So I built coldstart — a zero-dependency startup profiler for Node.js that instruments Module._load, reconstructs the dependency tree, and shows you exactly where boot time goes. Full transparency: I used Claude pretty heavily while building this — for scaffolding the ESM loader hooks, generating the flamegraph HTML template, and iterating on the tree rendering logic. The core idea (patching Module._load with performance.now() bookends) and the architecture were mine, but AI absolutely accelerated