Cpulimit is a tool that is very handy to have when doing data processing and other time consuming tasks. What it does is that it limits the CPU utilization of a process/program to a certain amount.
Imagine you started a cpu-hogging process and needs resources for other things
$ cpulimit --limit=50 --pid=1234
That will attach to the process and limit it's usage of cpu cycles to 50 percent of the available.