Thanks, since I rolled back the updates I haven't had a problem. I suspect that the Nvidia is the problem.
Or it was something else in the update. I'd vote for the video driver, except that drivers don't spend time churning the disk. I have a rather cynical suspicion that some sort of DRM code might have a hand in this.
But that said, I never install updated nVidia drivers these days. Too many things break, and often nothing is fixed. I have occasional system freezes that I think may well be linked to the video drivers, but the things I've tried to download in the past were worse, so I gave up.