[marginalia_nu]

Dithering is fairly expensive operation. Please don't do it client-side each time a resource is fetched.

[WithinReason]

It's much cheaper than decompressing a JPEG

[marginalia_nu]

Compressing, perhaps. But decompressing? Not really.

[WithinReason]

Floyd-steinberg dithering is at least an order of magnitude cheaper than JPEG decompression

[marginalia_nu]

Maybe if all else is equal, but let's not forget you'd be doing the dithering in javascript, and the JPEG decompression is implemented in a low level language, in practice probably hand written assembly (as performance-critical compression code often is).