The summary tab of a plan shows a useful chart called "Build Duration per Build"
This chart often confuses me because the y-axis, time, starts at the minimum time recorded, rather than zero. While I can see that this allows more detail to be shown on the chart, it can be very misleading if you don't look at the axis. I often see it and think a build has taken 10 times as long as normal, when in fact it's just a couple of extra minutes because the graph starts at an hour.