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).
Compressing, perhaps. But decompressing? Not really.
Floyd-steinberg dithering is at least an order of magnitude cheaper than JPEG decompression
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).