You could try using
renice with the application/process, which allows you to adjust it's CPU priorty. There are several applications that can assist in doing this, or you can do it from the command line in Terminal.
In Terminal the syntax is:
# renice ## -p PID
where ## is priority (value from -20 to 20, -20 being HIGHEST). PID is the process ID of the application. (For more information # man renice)
So if I open Activity monitor and notice Parallels is process ID 1222 and want to lower it's burden on the CPU I would use something such as:
# renice +5 -p 1222
Several Applications (that I know of) also have the ability to do this too: