If you have a multi-core processor (most people do these days) set the environment variable below.
e.g. in Linux in your .profile set on a 4 core processor..
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:
Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".