I recently switched from Unity 2018 to 2019 and noticed that sometimes Unity would freeze after script recompilation for a couple seconds in some of my projects. I couldn’t find anything on this topic, but I finally figured out what was causing it.
I sometimes switch between different custom layouts in the editor and some of them have the Package Manager window pinned in there. Depending on installed packages, this was causing Unity to freeze for extra 3-12 seconds after each recompilation or build.
Some packages seem to be responsible for most of that freeze time - for example ProBuilder, ProGrids or FBX Exporter. With default packages installed it barely takes any additional time, though it’s still noticable on my PC.
These freezes only happen when the Package Manager windows is open or pinned in the editor, closing it completely eliminated the problem. Hope this will help anyone with this issue, and that Unity Team will do something about it 