This is a known and very annoying issue that we have no fix for it
here is an extensive thread with various reports and some possible solutions that may or may not work for you
Lots of busy... Hold on.... etc? page-10
here is unity dev asking for feedback and bug reports related with this issue. If possible please do what they ask there maybe we can finally manage to fix this damn bug
Hi!
We are interested in collecting reproduction projects for any time the Editor unexpectedly stalls. On Windows, this shows the blocking Progress Bar Dialog.
To gather this data automatically, we are adding analytics information to report the causes for all stalls. However, until that is ready, if you would like us to investigate/fix any specific scenarios, this is a call for bug reports!
There are a few “expected” stalls, such as making a player build, importing assets, entering playmode, …