Thank you for these comments. I wondered about my keyboard too, but I noticed that it's only Mathematica. I opened the task manger to watch my CPU load - not related, but I noticed periodic burst of high GPU load, just when the notebook window was really lagging. But I was just typing ...
I work on a big monitor connected to a laptop. The two screens work and different scales. When I drag a notebook from the laptop screen to the attached monitor, the text becomes huge during the transition, and sometimes I have to wiggle the notebook back and forth to get the text back to the appropriate size.