Yeah, read up on the TimeDelta article. Motion blur works by actually boosting the framerate and drawing, say, 5 screens (for 5x) and merging them each time the screen is updated. So instead of events running maybe 75fps normally, with 5x motion blur it could be 375fps. In this case you have no choice but to use TimeDelta.
I don't think anyone appreciates how important it is! Use TimeDelta! (/goes to put it in sig...)