You are having the exact same problem as me and lots of other frustrated Windows users have had since C8 was released. I agree it is a showstopper. I’ve been in touch with Fabio B (who is a very nice and helpful person) at Steinberg over this. If I understand it correctly they seem to be working hard on it at the moment - although they seem to be having a surprisingly hard time pinpointing the issue.
For me the problem became even worse when I updated to 8.0.30 or 8.0.35, so I’m still at 8.0.20. Did you try the latest updates at all?
You might have seen my posts in the thread Lucas is pointing to, but just to conclude, here are two videos that I made to show what’s going on over here:
Cubase 8.0.20 compared to Cubase 6.5.5: