technically Series S could even do 4k60fps. It all depends on the game, some others even 900p30fps may be to much.
The PS3 can do 1080p/60fps, the Switch can do 1080p/60fps but Breath of the Wild runs at 900p 30fps and plenty run at 720p 30fps.
Ok, so this is getting to crux of what I'm asking. I was under the impression that the Series S is powerful enough to give the best possible performance at 1080p, and that the extra TFLOPS, RAM, processing speed, etc of the Series X is wasted in 1080p. So if a game is designed to run at 900p/30 it can potentially run at 1080p/60 on the Series X? Is that possible? I thought if a game was designed for a specific resolution/fps it can't go any faster no matter how powerful the console.