What an odd problem.
It seems like the Nintendo Switch seems to have a certain bug where, if you attempt to play a game with the Wifi on, it causes slowdowns and performance issues in games for some reason. Nintendo Life is reporting that according to an anonymous Switch developer, the Switch seems to drain GPU resources when Wifi is on in the background.
Moreover, turning off your Wifi seems to help with the problem. Turn off Wifi, or turn on Flight Mode, and it looks like game performance goes back to normal. It’s a most puzzling problem, but whatever it is, hopefully Nintendo can fix it right alongside its promised fix for reallocated GPU performance.
It’s a bizarre flaw, but the Switch seems to have launched alongside a lot of them- hopefully they can be addressed with time, because we feel the Switch is a well made machine with a lot of potential.