During a meeting with Nintendo at PAX East this year, we were able to confirm the docked resolution and FPS performance for Mario Kart 8 Deluxe on the Nintendo Switch. We spoke directly with Nintendo.
When docked, Mario Kart 8 Deluxe will run at 60fps in 1080p. It will hold that resolution when docked no matter what. When split-screen with two players, the game will maintain that 60fps in 1080p performance; but, it will drop to 30fps in 1080p when three or four players come in for split-screen local play.
We were unable to confirm the FPS performance for the game when it wasn’t docked.
Article source: https://www.technobuffalo.com/2017/03/12/mario-kart-8-deluxe-fps-resolution-nintendo-switch/