Not sure what your system is like now.
Or what your budget would be for upgrading, if you decide to.

I'd suggest posting on the forums at www.isboxer.com
Lax would have the best chance at figuring out your issue.

I've got my slave windows limited to 20 FPS and the main to 60 FPS.
Fef had a post within the last few weeks, where IS Boxer would lower the settings on a slave toon and raise them on the master toon.
So, my video settings are very low for all of the slaves (4 toons, on each team), but are at moderate levels for everything else except maximum on view distance for the master of each team; and when I switch characters, the high/low settings are applied.
Low settings on the slaves, especially shadows and view distance can help out a lot too.

I'm running with each of the physical cores, on all game clients.
And none of my logical cores assigned anywhere.
Not assigning the logical cores, was a very slight performance boost for me.