Was a 2x speedup for dithering even important at the time, especially if it involved a sacrifice of quality? It's not like dithering images was something people did regularly, in the first couple generations of Macs.
You'd dither a few images for a game or something you were building, that you were lucky to get from a scanner. It was a pretty rare thing. Speed wasn't really relevant, as far as I remember.
I only used a B&W Mac a few times, but I do remember Windows 3.1 doing on-the-fly ordered dithering when running with palettized color (and being very surprised at NOT seeing the dithering on the blue gradient of a setup program once I started using high color modes). Windows 1.0 apparently was capable of doing it as well.
Do you have a source for that? That's very much the opposite of what I remember. If you had 16 colors or even 256 colors, I don't remember anything in the Windows UX being dithered. Like I don't think you could pass an RGB color to GDI to draw a light pink line and it would dither it for you.
The only dithering I remember was indeed the background of blue gradients in Setup, and I always assumed that was custom code. After all, it's not like GDI had commands for gradients either.