symbionic relationship...
If it does get replicated to another machine... nearly cinches it.
If it don't, might be a Video setting of his... I know, that alot of slow-downs in the past have been mostly reated to a bogus video setting, or an "Over-Clockers" kind for the Video cards...
That is where I'd start... maybe downgrade the video-card driver or maybe even Update it. Might take care of it, but it might be tied to his profile worse then that.
Also, try and make a "new user" with a copy of his local profile... see if that problem migrates also. Worth a shot.
Does it does this with Remote Desktop too?