Does this problem affect any project you work on, or just one in particular? If you start a new empty project and add e.g. a single player with a piano, is the performance already bad?
We may be able to diagnose what’s going on if you can send me by email (to d dot spreadbury at steinberg dot de) the output of sampling the Dorico process:
- Run Activity Monitor (from Spotlight, or from Applications > Utilities)
- Find Dorico in the left-hand column. (A quick way to do this is to type ‘Dorico’ in the search box at the top left of the window.)
- Select the line that says ‘Dorico’.
- Click on the little cog icon in the toolbar.
- Choose ‘Sample Process’ (the first entry) from the menu that appears.
- Wait a bit.
- The sample should appear (a bit window full of text, beginning with the words “Sampling process”.
- Click ‘Save…’ at the top right, save it somewhere.
Attach the file to an email to me, and we’ll take a look as soon as we can.