It is not an official and supported solution, but if you don’t use video in Dorico, you could completely remove the videoengine.bundle from the Dorico application, since it is known that this engine causes a lot of cpu load, even if your project contains no video at all. See this post from Ulf:
In my case it makes a big difference, the first screenshot is from playing back a project with the unmodified Dorico application and the second screenshot playing back the same project but with removed videoenginge.bundle:
