Daniel -
I too am experiencing significant slowdown. My problem appears related to the size of my project. If I take a subset of my piece (first 3 pages) everything works fine, but it takes anywhere from 10 to 60 seconds for my screen to refresh when working with the full score. Given that my piece is scored for a quartet and is only about 200 bars long, clearly this should not be happening. I can continue work by splitting my piece up, but that is obviously not a good solution.
I have tried the usual fixes - restarting Mac, closing and opening Dorico - but no luck.
I am running late 2012 Mac Mini, Sierra 10.12.6, with 16 Gig
I will e-mail you files. Hopefully I am doing something dumb.