No, it doesn't affect loading time at all, not in the game and not in the editor.
A tile is only 4 bytes more in the palette. And even if the texture of an unused tile is being loaded too, that's nothing compared to the other stuff that has to be loaded.
Ive noticed when loading the edditor with tiles added using WEU that the extra tiles textures have to be loaded "specially". (takes ~1/4 of a sec to load em) what would this "be about"?
It probably affects loading time but just a little, so much that you wouldn't note it . Completelly worthless decrement compared to what widgetizer can do.