The computer isn't running slowly. It's crashing. More often than not, it's my browser that crashes, but if that happens enough times, the computer itself crashes.
Yes, I've also come to believe that the computer is trying to finish updating the last build before it downloads build 1903.
HAve you tried resetting your browser? You can usually go into your browser settings and find a “repair” or reset to default option that could probably fix your problem. Back up your bookmarks/favorites first though.
I’d probably try it with the Edge browser first to see what happens. If you haven’t cleared the cache in your Mozilla browser in a while, you should do that too.
I use Opera browser most of the time and have had pretty good luck with it. Pale Moon browser is also an option. It’s Mozilla based but without all the bloat.