If you have a multi-core processor (most people do these days) set the environment variable below.
MAGICK_THREAD_LIMIT
e.g. in Linux in your .profile set on a 4 core processor..
export MAGICK_THREAD_LIMIT=2
You will then only use max 50% CPU. For a 2 core processor set it to 1.
This should work in Windows too but you will need to use the Windows method of setting environment variables.
This link explains how:
https://kb.wisc.edu/cae/page.php?id=24500