having a system where you hold 2 items at once would be a pretty massive change to include in what is just a port; it's probable that it's based on
Mario Kart 8
, especially because it just makes sense for Nintendo to reuse anything from the Wii U that they can, but I think it's probable that it'll be a new numbered entry.
the apparent decrease in performance in
Breath of the Wild
is weird to me, because Nintendo really doesn't have any incentive to deliberately make their game look like it's running slow, and that would be an obvious thing to fix in post production of the video. the handful of times I've watched the footage, it's looked more like the slow downs are tied to hitting enemies in combat, and I seen to recall that being an aesthetic choice present in the Wii U version of the game. I'd have to rewatch multiple videos to confirm that, though, and I'm lazy, so I haven't done that.