Your slaves should not be getting a higher FPS then the master/main screen.

If you open up IS BOXER Toolkit.
Expand Character Sets, in the top pane.

Then check the lower left pane.
Click on the 1, 2, 3, 4, 5 etc, under Slots.

You can see what you've set, if anything, for maximum frames per second.
This is the screen where you can set which cores handle which accounts as well.

I personally run best, if all physical cores are on all games, and with the logical cores not used for any game.
I believe I have my slaves set to a maximum of 20 FPS, with no limit on the main.