Probably because the fill bucket is destructive and that preview is non destructive. i.e. using that preview won't eliminate your redo buffer, but using the fill bucket will.

It's doing it line by line in the preview so as not to use a lot of memory while not actually changing the image itself. If you hit OK (or pressed enter), the preview generation would be interrupted and the change would be instant (or at least not very slow) just like the fill tool because OK commits to an actual change rather than a preview of one.

Shorter: You'd basically need two copies of the image in memory to non destructively preview at the same speed. Or I guess a second undo stack which could be applied on cancel...

how big is that canvas?

I'll try to improve the performance of filters (including Replace Color) for the next version.

Amazing work. Aseprite is now killer on Mac.