After the 1.18.15 update, my performance had improved significantly - although many non-performance bugs remain - and I was quite pleased. However now I can fly smoothly for the first 20-30 minutes, but then I get occasional freezes, and twice FPS on approach dropped all the way down to 5 or 6, rendering the game unplayable.
In so many ways MSFS 2020 is the best simulator I have ever run, but the recurring bugs and performance issues, apparently related to practically every update, are hard to understand. I have not made any changes to my system configuration or in-game graphics settings since 1.18.15.