I’m not 100% up to speed about VS compiling process or compiling processes in general but if I had to take a stab in the dark I would think that modules can only be compiled with a single thread. Also, I believe modules have to wait to compile until all of their dependencies are compiled. This means that your project needs to be compiled in a certain order and wont always use as much processing power that is available to it.
When i am compiling. Most of the time processor is IDLE. Only sometimes it is busy 100%.
I want it to be 100% busy all the time ;d. All cores.