YouTuber "Will" uploaded a video where you can see Mario Kart 8 running at 8K, using the latest version of the WiiU emulator, CEMU.
Will used an NVIDIA GeForce GTX1080Ti to be able to run this game at 8K (with supersampling) and as you can see, the game runs very fast even in such an resolution.
Mario Kart 8 runs at 720p on Wii U and on 1080p on Switch. And only this difference in resolutions as the speed at which the game runs, shows the great work CEMU developers do.
The game looks amazing. Even with the resolution Youtube allows us to see it, it looks very beautiful with a very fine resolution, as it should be from the beginning.
Also see the difference has the resolution in games, is this MetalJesusRocks' video, where he shows us various GameCube games with different resolutions.