@gauntletlover - Thanks for the reply, I'm using "BigBangBar PHYSICS 2.5" from "pinsimdb.o*g/pinball/table-5237-bigbangbar" (Sorry for the malformed URL, I hear that there are some rather childish rules about posting links to other sources around these parts)
@ndz: I'm using an old AMD Dual Core Opteron which is older than an Intel i3 without much impact on performance, if any. I'm using a Geforce 560 on Windows 7 x64 for the play field and backglass and a older geforce 6800 for the VGA DMD. If you aren't already, try to run each screen at their "native" resolution, I noticed that improved performance significantly on all screens. For example, I can drive 1900x1200 on my backglass screen, but the native resolution of it is 1366x768. There's no benefit to driving the screen at a higher resolution since the backglass images don't benefit from it and going higher only increases the CPU and GPU processing demand. So with that said, try that on your USB based VGA interface. You may be driving a high resolution that's saturating the performance of the USB bus.