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).
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).