This is normal, has been expected since Unity was created.
It takes time to capture the window from Windows and than 'draw' that in a Mac window.
So when you update/move the window, we have to find the new location inside Windows, track the changes and re-draw the UI on the Mac. You can sort of see this in the Thumbnail view where Windows has this grey background. That's what Windows 'thinks' it's doing.
Remember too that operating systems aren't supposed to do this
So generally we look at it like this...
Are folks using the window when it's being dragged? No.
As such, we put our engineering efforts on making it perform when it's not moving, rather than try to solve for this.
It's a perception thing, and we've argued about it at length. "It feels slow when I drag the window!"
But spending resources so the UI doesn't 'tear' when dragging the window around is much better spend in just about any other area of the app.
Thank you for the answer! Knowing this is normal is something of a relief, since now I know there isn't a problem with my installation!