I think I found the problem but I don’t really understand the impact of the solution.
Basically I used the command grep . -r /sys/firmware/acpi/interrupts/ as seen on How to handle cpu interrupts that causes high cpu usage , since it described a problem exactly like mine.
I found out that one of the interrupts had strange values (others were 0):
/sys/firmware/acpi/interrupts/gpe6E: 8565531 EN STS enabled unmasked.
I executed echo "disable" > /sys/firmware/acpi/interrupts/gpe6E as root and indeed it worked!
I have got massive improvements in htop:
But I want to know, does anybody know if there any side effects to disabling this interrupt? Is there a better way of doing this?