Unlike Lite, the regular Switch has increased performance with the dock in TV mode. Most games for the Switch are made with the dock's increased output in mind, while making use of resolution scaling and other downgrades (like no anti-aliasing) for handheld mode.
SwSh being made for handheld primarily means they are focusing on what handheld only can handle, which is why SwSh lacks a lot of detail and foliage aside from the tall grass where Pokémon spawn from in the maps shown so far.
For instance, BotW is made with the dock in mind, which is why when you go from TV mode to handheld, the game performs noticeably worse and the resolution scaling up to 360p kicks to keep the game at playable framerates. I doubt SwSh will make use of the scalable resolution, instead, they build the maps in such a way that the Switch can at least get 20 FPS in handheld mode, this is also why the draw distance in the Wild Area is very low. It's also why any other map has a fixed camera; not much has to be rendered as the player won't be able to move the camera in such a way that a lot has to be rendered, putting more strain on the Switch, and also they'll be making full use of culling that the player will not see.
I hope this explains it a bit.
