Silence mode do actually do a lot of difference. Saving now takes estimated 0,5 seconds (of stalling/everything hangs, opposed to the other 5,5 seconds you mentioned).
I’ve stoped using my MIDI keyboard since yesterday, which seems to do just a little on the positive side.
Smaller window has no impact at all.
In short: it really seems to me the stalling/that everything hangs has something to do with the playback. In silence mode I now do not get any spinning wheels, but it does stall for 0,5-1,5 seconds when saving.
Where I am at in the project has nothing to say for the stalling. I can select something, scroll (I use my bluetooth mouse, cmd+scroll and shift+scroll), save, or start playback. I think that is all the situations where the lag happens.
Since I am on a deadline for tomorrow, I will work mostly in silence mode now. But for later, and maybe for other users, it would be fantastic to try to find a fix for this. I really enjoy working in Dorico, when it just works.