How can I find figure out why a machine has consistently 70%+ CPU usage?

My machine (Macbookpro, Archlinux) has consistently a CPU at 70%+, yet I have no clue why. This doesn't happen on MacOS, only on Linux.

Using htop, I can see no program using more than 2% of CPU. Even without X and after killing every running process I could find, the CPU usage is still at 70%+.

Thanks!