Despite what the previous poster says, Benchmark::cmpthese uses CPU time to create the chart with ratios. By default, Benchmark::timethese shows a breakdown in times (wallclock, system, user, child-system and child-user), but doesn't give you a ratio chart. The iterations/second given by timethese will use CPU seconds. You have some control of which times will be shown by using a third parameter to timethese.

Do mind Benchmark wasn't written with wallclock seconds in mind (and in fact, it wasn't written for the purpose of measuring I/O). You're probably better off using some other tool.