The described behaviour is normally associated with the graphics drivers more than anything else. Unless of course your settings are just starving the whatever of resources.

It would probably help to know a few things, such as:
What game are you referring to (assuming this is a multiboxing game type thing).
What hardware do you have.
What monitoring have you done on the hardware (e.g. OpenHardware Monitor/HWInfo/TaskManager etc).
What settings are your games using.
What software, if any, are you using to help you multibox.

There is probably more, but it should be a reasonable start.