Tested 9.0.30 video engine out of curiousity - empty project just a video to playback in Pro.
Video file playing at 36%(peaks 50%) cpu usage in Cubase video player.
This as thumbnails are done processing.
Playback is a bit stuttering as well - 25 fps 1920x1080p. A MOV with H264 inside with ALL-I from a Canon 80D - no audio.
Removing that video track and insert VST instrument track with VidPlayVST64 plugin - use 9%(peaks 20%) cpu.
Playback smooth as anything - from first frame.
So Cubase running this VST instrument and playing back same video keep computer performance much better.
I matched video window size for actual video to be similar.
Just some feedback that there are things to optimize quite a bit in new video engine.
Check out VST - http://vidplayvst.com
My computer specs are in my signature.