You cannot really choose a different CPU speed for each app on your system - the CPU has to run at a constant speed to suit the entire system demand or to save power for example.
What you can do is assign the priority each application on your system gets, which in turn will decide how many CPU cycles each application has dedicated to it. Check out this app:
Surely that will just hamper performance as a process might suddenly require 75% of CPU cycles, but you've only allocated 33% max to it for example?
OS X does a good job of assigning CPU time to different processes as it is and there's little chance of you speeding anything up, just slowing things down.
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