This has to come down to memory bandwidth, right? If portable mode is identical and simply at a 720p, the clock increase when docked should be plenty to run at 900p and have identical performance.
I would say optimization issues from porting a game designed for Wii U, but MK8 goes from 720p on Wii U to 1080p on Switch, with a more stable framerate to boot.
Confusing.
Worth considering that the WiiU has a unique memory setup that Zelda may be utilizing to it's fullest to run. There may be additional changes and optimizations required for the game to better take advantage of the Switch. As an open world game though it definitely has a lot more going on that Mario Kart does, so it may not be a fixable issue, or one that can just be patched. The game may need to be ground up for the Switch to really take advantage of the system.