You almost never want to use average as your metric when dealing with time. For time, the 95 or 99 percentile latency is going to be measure of how many requests out of a hundred a less than your metric. Esp when the thing you are measuring is a piece of larger system, which is the crux, because of how a resource in turn requests k-more resources. Each request has an equal chance of being over the latency percentile. Even at 10 requests, only 90% of sessions will be under the 99% latency.

See also, Gil Tene https://www.youtube.com/watch?v=lJ8ydIuPFeU

A nice overview, http://bravenewgeek.com/everything-you-know-about-latency-is...

See also, http://highscalability.com/blog/2015/10/5/your-load-generato...