Using all CPU power to improve processing speed is very much intentional, it is playing matches using as many cores as it can.
If you are doing other things on your PC at the same time and don't want FM to takee such priroity you can either do what you did, but persoanlly I'd recommend lower the priority of the process slightly, that way if you aren't doing anything else FM can run as fast as possible, but if you are it will take secondary prioirty and should allow other apps to proceed without a hitch.