This:

In T389863#10703782, @Daimona wrote:

Debugging the performance of a request that takes 2 seconds isn't easy when the log resolution is 1 second, but I tried.

… prompted me to think that it'd be great if MW via a flag, or maybe by default when in development mode, wrote debug logs with more time granularity. Looking at our code it seems this could be a fair bit of work, however.