Select your Character Set in IS Boxer and then select Slot 1. Then go to Performance tab and select all Cores(must be blue).
Repeat it with every other slot.
That's what the CPU Strategy Wizard is for, you don't have to go to each slot manually to do this

This pretty much sums up the conversation, have you been here? http://isboxer.com/wiki/HOWTO:Tweak_your_framerate