There is currently an issue with unity that causes these loading bars that freeze unity for minutes or hours. there is no fix.
We don’t know what is the problem that cause this behavior. Some have different ideas and speculations you can see some here in this thread
Lots of busy... Hold on.... etc? page-9
I’ve had the same problem you describe and it will only get worse from now on. Doing random things like saving a scene or existing unity or duplicating an assets lock unity.
Some days it will work fine others you will get these loading bars very frequently and you can’t do your work.
It got so worse that I’ve had to scrap the project and slowly rebuild it as modules in separate smaller projects so if this issue reapers with any of those projects will take less time to rebuild or maybe I can identify what is the cause.