Game is CPU heavy because of all the drawcalls. Even if your CPU doesn't seem like it's working, it's bottlenecking somewhere. I wish I knew, but I'm not a programmer. What I can explain is that modern consumer software that monitors your resources on the PC only show you the total capacity of the CPU, it doesn't segregate any kind of particular hardware.

For example, you may not be bottlenecking on Floating Point, but Integer is completely saturated and slowing everything down, or even something like Network I/O, Memory controller and even Cache, kicking things out of L3 into system memory.