Traditional serverless functions (AWS Lambda, Vercel serverless) spin up a Node.js process inside a container on first request: this takes 100-500 ms. Edge functions run in V8 isolates at the CDN PoP closest to the user. Isolates start in under 5 ms because they share the V8 engine that is already running; there is no container to provision. The trade-off: edge functions cannot run Node.js-specific APIs (file system, native modules), have tighter CPU and memory limits and run inside a restricted sandbox.
The limitation that matters most in practice: no Node.js built-ins. You cannot use `fs`, the Node.js `crypto` module or any native addon. You get the standard Web APIs: `fetch`, `Request`, `Response`, `crypto.subtle`. If your existing serverless function uses any Node.js-specific API, a direct port to edge will break silently during build or at runtime.
Vercel: Edge Functions
Covers the Edge Runtime API surface, limitations versus Node.js serverless and when to use each.
https://vercel.com/docs/functions/edge-functions