There's no GUI option to toggle it in the default UI. There's a console command to turn it on and off (don't know what it is off-hand) or various addons have options to toggle it (HealBot and Option House for sure, possibly PerformanceFu and/or ACP and others as well).
this just in: I use an addon manager aptly named 'Addon Manager', and at the top center of its GUI is a check box for CPU Profiling.
It doesn't say what that does, what its function is, and offers no way for me to evaluate whether I should use it or not.
CPU Profiling tells wow to add code to every addon (code that records timings) so that wow can more accurately tell which addons are using the most CPU time/clock cycles.
If that explanation doesn't make you understand anything, imagine you have 100 addons, and your performance is sluggish. You know that some of your 100 addons are causing this sluggishness, but you don't know which ones, so turning on CPU profiling and using an addon that retrieves the associated data that wow records for each addon will show who these addon culprits are.
The problem with CPU profiling obviously is that wow inserting the code to record CPU usage will obviously cause even more sluggishness (lower fps) and make the problem look worse than it really is (wow updates the recorded data about once a second). But anyway, CPU Profiling is reasonably accurate given that all addons are more or less equally affected by it, and you can be assured that Pitbull/Cowtip are indeed using a big portion of your computer's CPU chips.