IA64

There is a huge difference in cooling methodology between an iMac and a Mac Pro. The iMac runs a consumer CPU that is soldered to the board, and  the higher grade cpu's often has the boost clock artificially limited to prevent throttling due to insufficient cooling.
 
The Mac Pro runs a user upgradeable server grade cpu. The Mac pro has a pretty ingenious cooling system that runs 3 giant 20cm fans that are acoustically optimized and ducted though a proprietary stacked fin heatsink. The Mac pro's cpu is in no way limited, running at the standard Xeon specs as indicated by the CPU. And while it has great acoustic properties, a high spec CPU running under sustained load will produce an audible fan noise.

The 3970X runs at 3.7GHz base clock speed with up to 4.5 GHz boost clock...
The 3900X runs at 3.8GHz base clock speed with up to 4.6GHz boost clock...
Apple's 28 core Mac Pro 2.5GHz base clock with up to 4.4GHz boost clock.
The source is AMD's and Intel's web sites:
https://www.amd.com/en/products/cpu/amd-ryzen-9-3900x
https://www.amd.com/en/products/cpu/amd-ryzen-threadripper-3970x
https://ark.intel.com/content/www/us/en/ark/products/193752/intel-xeon-w-3275-processor-38-5m-cache-2-50-ghz.html
 
Base clock is a guaranteed all core clock speed while boost clock is a potential clock speed for a single core.
 
Edit:
A clarification about my description of boost clock:
The definition for boost clock i wrote is for the "Max Turbo/boost Frequency/Clock" CPU manufacturers list in specifications.
You will get an all core boost clock but it will be slightly higher than base clock and nowhere near the max boost clock (Unless you use extreme cooling such as LN2,DICE,etc).