Unity freezes (5+ min) after version update. How to investigate the problem?

Hi,

I recently updated my project from 2018.4.14 to 2019.4.12. After this, Unity started freezing for a good 5 minutes whenever I save my scene, or enter/exit playmode. So I made a new project and rebuilt everything in steps, but it still keeps freezing from time to time. It makes me really anxious every time I want to save :(

Does anyone have an idea of what this could be? Or what I could do to investigate the problem? The editor doesn't give any warnings or errors, so I don't know where to start looking.

Thanks for any help!