Author here :) There is no trick here actually. It's just HTML / JS / CSS. Browsers are pretty good at compositing CSS transforms - and know how to handle DOM updates coming from event handlers & requestAnimationFrame. There is one CSS hint that speeds it up a little "will-change: transform". Another important ingredient is to update the object position in the (pointermove) event handler - so that the responsiveness is low - physics engines also could do this, but it's usually an overkill.

You can take a look at the script in the website's sources - it's inline.

The trick is to use a game engine or something similar (not vanilla CSS/HTML). Flutter web would work well here, once Safari's WebAssembly GC support gets merged, you will have a lot of options.