I used to wonder this in 747, 757 and 737. When you deviate, it doesn't seem to start going off very rapidly and seems to maintain a steady profile. I suspected it continued using a 'direct to' dtg rather than the equivalent magenta dtg, though nobody seemed to have any idea. I did find the 737NG VNAV descent used to make some sudden peculiar and unpredictable speed reductions for no reason that I could fathom, even when high on profile.