Why is the System process using 40-60% of CPU power all the time?

I decided to think a bit more hardware-related so opened to case to clean the processor heat sink. There was hardly any dust so that definitely wasn't the cause, but now System only uses around 1% of the CPU.

The heat sink must have not been fitted properly because all I did was remove it and put it back on and I didn't change anything else. Now my CPU graph actually goes down to ZERO when I'm not doing anything. Yay!

N.B. I'm an idiot.