The performance boost in the Switch version is really nice. The frame rate went from painful on Wii to just bearable on Wii U to super smooth on Switch (well except in crowded areas but then it still matches the best Wii U fps). This especially benefits battles, the game always had great animations for the enemies but seeing those animate so smoothly really is a treat for the eyes. Draw distance seems better too, less pop in, better textures, better lighting.
For some reason the Switch version won't let me take screenshots (the Switch OS ones, in game photos work as expected). Apparently they plan to allow it at a later point but it still sucks that it is disabled. I'm guessing PS4 allows screenshot and video capture everywhere?
Login is slow because it loads a webpage for that. Wii and Wii U had their own menus for the login that didn't need to be externally loaded. The password isn't saved and has to be input every time. They said they will fix this too but for now it's a bit annoying.