For example, because I was trying to use fine-grained timers for everything async, I thought the JSON parsing library we were using was a bottleneck, because I saw some numbers like 30ms to parse a simple thing. I wasn’t measuring total throughput, I was measuring individual items for parts of the flow and incorrectly assumed that that applied to everything.

You just have to be a bit more careful than I was with using timers. Either make sure than your timer isn’t going across any kind of yield points, or only use timers in a more “macro” sense (e.g. measure total throughput). Otherwise you risk misleading numbers and bad conclusions.