Hey,
You should give this vid a try:
The guy explains step by step why a more powerfull system doesn’t mean a better realtime performance necessarily and why. A quick shortcut to the long video is to suggest that a decrease in performance means that the CPU is getting locked up by one ore more processes or a device somewhere in your system. He also shows you the way how to analyse the problem and how to track it down. Such an issue is most of the times system specific, so it’s hard to say this or that when you do not have the system in front of you. It might even be something that is completely unrelated to cubase or the audio card, f.e. something in a usb port like an external drive. It might f.e. even be the type of port used for a ssd.
Try to follow the vid focussed, since with all the drawings it looks a bit odd, but in fact he goes all the way and very fast to the core of the topic. If you have few patience, the examples given start at minute 17.00.
FWIW
kind regards,
R.