Attached screenshot: Me trying the same function on a command block being powered by observer clock. The command worked as expected and I got the right map.
Just realized the "zoom" was at 3. Set it to 1, and it loaded instantly. Maybe the zoom is the problem here and not the styling, or maybe some sort of combo.
May help you figure out what may be happening, or to mark this as a resolved issue or duplicate.
Tried in 20w17a with zoom of 3, took about 18 seconds to generate the map. zoom of 1 is still near instantaneous.
The zoom being 3 might be an issue I think is not worth solving. I think if zoom of 1 works fine, it is probably a fine issue.