Tracing the Express Middleware Nobody Talks About: Compression

When we talk about observability in Node.js , we usually trace things like: HTTP requests Database queries External API calls But there’s a hidden performance layer most tracing setups completely ignore . 👉 Response compression Many Express applications use the popular compression middleware to reduce response size and improve latency. But here’s the problem. Nobody traces it. Why Compression Deserves Tracing Compression happens after your application logic finishes but before the response is sent to the client . That means: HTTP Request │ ▼ Express Route │ ▼ Application Logic │ ▼ Compression (zlib) │ ▼ Response Sent If compression is slow, you may see: slower response times increased CPU usage delayed response delivery But in most observability setups, this time is invisible . Your traces might show something like this: HTTP Request └── Route Handler └── DB Query But you never see compression time . That’s exactly the gap I wanted to close. Introducing Compression Instrumentation I bu