Edge Function Latency Calculator

Edge functions are serverless functions executed at CDN edge nodes (Points of Presence) geographically close to users rather than in a single central data center. Providers include Cloudflare Workers, Vercel Edge Functions, Deno Deploy, and Fastly Compute. They reduce latency by eliminating the round-trip to a central origin and are ideal for personalization, A/B testing, authentication, request routing, and geo-specific responses.

Cloudflare Workers use V8 isolates rather than full container or VM instances. Isolates share a single V8 process, so spawning a new isolate takes microseconds rather than the 50-500ms typical of container cold starts. Cloudflare pre-warms isolates globally, meaning in practice Workers have ~0ms cold start. The tradeoff is a restricted runtime — no filesystem, limited memory, and a subset of Node.js APIs.

Percentile latency describes the distribution of response times. P50 (median) is the time within which 50% of requests complete — your typical user experience. P95 means 95% of requests complete within that time — covering most users. P99 (tail latency) means 99% of requests finish by then — the worst 1% experience, often 3-10x the median. Optimizing P99 matters most for interactive applications where tail latency affects perceived performance.

Simple functions (header reads, cached KV gets, routing decisions) take 1-5ms of execution. Medium complexity (JSON parsing, a single upstream fetch, lightweight transformation) typically runs 5-20ms. Complex functions (multiple fetches, database queries, crypto operations, large payload processing) can take 20-100ms+. The dominant cost for edge functions is often the upstream fetch latency rather than CPU time, since edge runtimes are single-threaded and non-blocking.

Cloudflare Workers: largest PoP network (300+), near-zero cold starts, but limited runtime (no Node.js compat without polyfills). Vercel Edge: tight Next.js integration, good DX, fewer PoPs (~90). Deno Deploy: native TypeScript/Deno APIs, fewer PoPs (~35), higher cold starts. Fastly Compute: WASM-based (Rust/Go/JS), extremely fast warm execution, higher cold starts, requires Fastly's SDK. Choose based on your existing stack, required APIs, and target geographic regions.