I must not be the only person who experiences this. When scrolling down or up there is a noticeable frame-rate drop. What is intended to be smooth ends up been jerky and my processor tries to figure out how to move the high resolution image while keeping the robot on the side in the same place.
I am using a 2017 macOS Catalina with a 2.5GHz i7 processor (no graphics card). I do not experience an issue like this on Math or Physics Stack Exchange.
Although I have tagged this with bug, it really is intended just the be a discussion on what can be done about the frame-rate drop. The background looks pretty, but I do not think it is worth the cost in frame-rate for some users. Is there any work which has already been started to solve this issue?
Edit: A recent answer has identified the issue. It is currently unclear to me how to setup my browser, to automatically disable the fixed scroll feature of the background attachment when I visit Worldbuilding S.E.
<body class=...element, then scroll through the "Styles" on the right and uncheck the CSS rule. To inspect element, you may have to enable developer settings in Preferences. $\endgroup$