If you look at a detailed graph of CPU usage you should see a pumping action as the CPU goes full bore and then stalls out waiting for data. Filling in those stalls is what gives you the extra performance.

What is your preferred tool for gathering this kind of detailed data?

On windows It's WPR and WPA. In sure Linux/Mac has similar system wide profilers. WPA will give you detailed wait analysis.

I don't have windows. I will google for "systemwide profilers". Thank you.

I doubt this will change anything, (because the compilation time decreased as CPU time was added) but the information could be helpful in other situations.