Earlier quoted context omitted.
With that kinda volume the 99th is not interesting at all. Should be looking at the 99.99th.
The latency to human user is what matters. If I get an answer in under 100ms as a user 99% of the time, I don’t care whether 100 or 100K times per day someone else hits that 1% worst case.
Also keep in mind how percentiles compound when you have more than one service involved in serving a customer request. For example, let's say it takes 5 internal requests to serve an external customer request and each of those services measures latency SLAs at the 99th percentile. The customer request may only finish inside the SLA 95% of the time (99%^5)