National,
Thanks so much for testing this.
As far as waiting for the clip to process, I'm not going to do that after moving each point. If that's required, I'll give up on masks or find another program. As I've been saying, VP should not update the whole clip after every little adjustment, but should wait until you're finished. Closing out of the effects window would seem to be a good trigger for the program to update the whole clip with your finalized adjustments. Seems like doing it for every little move you make is just asking for trouble, and I can't figure out why it was made that way.
Anyway, rant over... what I will try is to wait for it to finish processing before I move on to the next keyframe.
Also another thing I'm going to test out is turning off my secondary display. There's no actual 2nd monitor, but I run sound through HDMI to a receiver, which makes Windows think there's a second monitor (why after all these years you still cannot turn off a 2nd non-existent display when you're running audio through HDMI is another rant). I think I made this change somewhere around the time I started having problems, so it might be a possibility although I don't know why it would affect VP.
RE: the issue you discovered while doing the tests. I've never run into that, but the clips I mask I already have trimmed down to 10 or 30 seconds (or whatever) and made it into its own video before doing the mask.
BTW, do you have any problems when moving points? My issue is that some parts of the masks are really hard to see the points, and I end up accidentally creating new points. I found out that this is a problem because the keyframes don't simply save the shape of the mask as I initially assumed, but are saved point positions. If you add or delete points later in the clip, it messes up previous points because it's either taken away points (collapsing the mask in areas) or adding points (making sections of the shape squeezed and mushed together). Also I wish the function of the cursor changing to "move all" function could be disabled. I will occasionally accidentally move the whole mask and then have to move it back in exactly the right position.