The framerate drop is not caused by YouTube but by Cubase itself ! On the video we can see your pointer moving smoothly for a brief moment while in the background the editor window is still refreshing at 3 fps when you are zooming out.
Can you eventually double check the experiment ?
Edit : Added some additional information and removed the link to the video (first 10 seconds) because the Editor was in the VariAudio tab which is the only place in the editor where the issue does not occur, don’t ask me why, and the user repeatedly pressed G to zoom out, which explains the heavy “stutter”.