Scrolling the docs on a large screen (2560x1440) has a very noticeable impact on frame rate (drops to between 10 and 20 fps). This is due to using background-attachment: fixed for the body's background image. Disabling that property fixes the issue. Of course, that also disables the fixed background.

There is a solution presented in this article, but it's not clear to me how to adapt this to a background for the entire body.