Our delivery analytics page loaded every GPS point for a city into the browser to draw a heatmap. At 1.2 million points the tab froze for 40 seconds and sometimes crashed. How we measured where the time went, moved aggregation to the server with grid binning, and rendered the result with a WebGL layer.
After enabling React Compiler we removed our manual useMemo calls, and the order table became slower than before. The compiler had bailed out of that component because it mutated an array during render. How we found which components were compiled, fixed the rule violations and verified the result.
Typing in our search box felt sticky, with INP above 400 ms on mid-range laptops. The React Profiler showed the entire product grid re-rendering on every keystroke because one app-wide context held both slow and fast-changing state. Splitting contexts and a selector store cut that to a handful of renders.
Whenever someone exported a large CSV report, our Node.js API stopped answering everything else, including liveness probes, and pods restarted mid-export. Building 200,000 rows synchronously blocked the event loop for seconds. How we measured it and moved the work off the main thread.
Our reading progress bar stopped at 92% and saved scroll positions were always slightly behind. A hand-written leading-edge throttle dropped the final scroll event of every gesture. We added a trailing call, tested it with fake timers, and compared requestAnimationFrame and scrollend.
E0502 stopped us from updating parent totals inside a HashMap loop, so someone cloned the whole map to make it compile. That clone ran every ten seconds over 1.2 million entries. Here is why the borrow checker was right and the fixes that kept memory flat.
During a launch our nginx reverse proxy returned errors and logged worker_connections are not enough, at well under the expected capacity. Every proxied request holds a client and an upstream connection, and the file descriptor limit was lower still. How we sized it properly.