Migrate API Routes from Node.js Serverless Functions to Vercel Edge Functions

We are evaluating whether to move API routes from Vercel Serverless Functions running Node.js 18 or 20 to Edge Functions running on V8 isolates. The goal is to reduce cold-start latency while keeping existing npm packages functional.

Edge Functions do not provide Node.js built-ins such as fs, path, crypto, process, or Buffer, and packages with native addons generally require a compatibility layer. Cold-start measurements can also be influenced by regional routing and response caching, so the observable latency gap may not reflect the function runtime alone.

What criteria should determine whether an API route qualifies for the Edge runtime? What approach isolates cold-start latency from caching and regional routing before comparing runtimes? Which compatibility threshold should block migration for packages that rely on Node.js globals?