Basically each of the windows is rendered at full size with IS Boxer.
So it is the same as having five copies open, each with the same full sized screen.
I'd reduce the settings on your slave windows somewhat, to alleviate this issue.
Reducing the background FPS might be all you need.
But you can reduce the graphical eye-candy on the slaves as well, or even set it such that the current master has good settings, while the slaves have reduced settings.



You could try to limit your background frames per second.
So your slaves don't use as many resources.
IS Boxer Toolkit > Character Sets (Top Left) > Click your Set (Top Left) > Select each slot in turn (Bottom Left) > Check Limit Background FPS > Set to something like 20.



You could set up a mapped key to lower the graphical options on the slaves.
And to increase them on the master.
And make that part of what happens when you switch from one character to another.
http://www.dual-boxing.com/showthread.php?t=32654