Same issues here on an 27" 2560x1440 144Hz screen. It took me a while to figure out that it actually is a frame drop. First I thought that my mouse has issues. But if your monitor supports showing the actual frame rate, then you will notice massive frame drops down to 30-40 fps which makes using Cubase impossible.
So deactivating G-Sync seems the solution, but a genaral fix would be way more better.
By the way, I also recognized this issue when using HitFilm, a video editing programm. So Cubase is not the only “one” who has to fix something.