The boosts when a single core can run over clocked are less likely to be easily measured, but you can measure thermal throttling very simply with the thermal logging of pmset. With 8 cores on many MacBook Pro and dozens of cores on the iMac Pro - you're boiling a ton of complexity down to one number. Since the code and each core of a CPU can and will change hundreds of times a second based on ephemeral load factors, power optimizations that consider what's visible on the screen, what network data arrives, the idea that a modern CPU even has one 'common' clock rate at any one point in time seems to vastly over-simplify reality. CPU interrupts on macOS are shaped in intervals of 150 ms and much of this detail is public from WWDC 2013 and later on power management, App Nap ( Session 209 in particular is both good and approachable) and battery life optimizations on macOS. I have to think this is a bit of an X Y question in that 'What are you going to do once you get this number?' and want to answer that directly, but let's dive a bit into what you're trying to measure.

Terminal mac cpu info