I had the same issue a few days after updating my Fairphone 4. All window animations were suddenly slow. I’m using the default launcher.
I followed this tutorial, which solved my problem: https://www.zdnet.com/article/i-changed-this-android-setting-to-instantly-double-my-phone-speed/
All my window animations were set to 2x. After resetting them to default (1x), everything is back to normal.