Could you please post screenshots to illustrate the issue?
Does turning off »Use Graphics Processor« in the Performance Preferences and restarting Photoshop have any bearing on the issue?
When I try to modify an existing mask with the quick-mask-mode
Why would you edit a Mask in Quick Mask Mode instead of simply painting, editing, … it as the Channel it is?
I'm having the same problem and I just downloaded the latest updates, which didn't seem to fix it. Here's a screen shot:
And the red just along his tricep I had to draw "blind" because it wouldn't populate the brush stroke to cover his arm until after I moved my mouse to the side of the screen. Any changes or movement on the screen is slower than molasses.
I had the same problem, but for me it was resolved when I went to Edit->Preferences->Performance and then chose Advanced Settings (Graphics Processor Settings) and changed the Drawing Mode to Normal. If that doesn't work, you could try "Basic" as well -- but if you are having problems with the Quick Mask, Advanced Drawing Mode might be the culprit (presumably a bug with the way Adobe is using the GPU for the mask).
Also try updating your video card driver from the GPU maker's website (not Microsoft or a system builder).