I ran into a pretty big performance issue in a plugin when it created a mutex in the TS_EVENT_HTTP_SSN_START hook. This caused a significant delay in processing the request. It ended up delaying sending the request to the origin by between 50 and 100ms at the 75%+ percentile! Here is a sample plugin which illustrates the problem:
What is also bad about this performance problem is that it is not visible in any of the milestone metrics including ttms. It shows up as a delay between in TTFB on the client side. It caused a lot of confusion to not be able to see the latency in slow logs, thus this request to expose the time spent here to the milestones.