Comment by kevingadd

We accidentally had a regression slip into our TS once that made it take over 7 seconds to typecheck a file, and that was surprisingly painful to diagnose. It meant our CI builds were slower, our local builds were slower, and the language server (in VS code, sublime, etc) would just randomly go unresponsive while editing. If there were tooling to track deltas in that per-file we would have noticed it immediately.