Ok after playing around with this a bit, Digicheck is not the source nor are the rme drivers, though going back about a year did change things a bit (safety buffers in the drivers were adjusted perhaps?) Adjusting Digicheck's update rate helps a tiny bit, but I think this is related to Win7/Vista Aero & having an older app open alongside your DAW/audio app and digicheck. I can actually get even foobar2000's asio/mme output (direct to RME driver output so it uses the latency setting) to generate this effect just as easily as a DAW app, which means all 8 of my cores are essentially idle.
So using any nvidia 259.xx & 260.xx driver (which I was doing for reasons related to video editing software) I have an issue when there is both 1. a low latency audio app (or two) open with under 3ms latency and 2. an application open that makes legacy UI calls and takes the legacy (cpu) compositing path/buffer under Aero before being composited on the gpu with the 'current' software. Closing that 'older' software immediately results in a return to responsiveness for the UI, as of course does disabling Aero entirely thus forcing the entire desktop into the legacy compositing path/mode (ie, on cpu.) Personally I prefer not to do that, but I can report that this seems to have nothing to do with RME's software, at least for me...