Just a quick update: The presonus support team gave me some good pointers on how to optimize my Mac so I can improve performance. Disabling the network card, repairing disk drives, and resetting the NVRAM. Worked really well. I'm just amazed that my performance meter on Cubase didn't show that the Mac was stressed. What good is a performance meter if it doesn't report when there are problems!
Now I have my next mess to tackle: projects are crashing on load-up. I think I have the problem isolated down to one plug-in. I guess it's time for a re-install of Drumagog!