First off navmesh.
Basically smaller tile sizes are increasing update times significantly. Going from 256 to 128 about doubled the time. Tried 64 and using async update after a couple of minutes it hadn’t yet completed. Stopped playing and then Unity crashed at that point. This is on a larger 4k terrain. I didn’t test on smaller terrains but I was using 2k terrains in 2019.2 and didn’t see this behavior. Smaller tiles with everything else mostly equal outside of the terrain being smaller were comparatively faster to rebuild then larger ones.
Terrain holes. Periodically it’s creating holes larger then the entire area that I’m fetching and then setting back. Like 2x to 3x larger. And my logic doesn’t even set what I do fetch to all be a hole.
Last thing is textures are not saving in the same way as previous versions. If I write to a Texture2D and Apply I see the changes. If I re import or restart Unity it reverts. Maybe there is an extra step here that’s required now?