Over at VGC, the team previewed the game recently, where they were greeted with two gameplay modes as part of the demo. These were a 4K / 30FPS mode, and a 1080p / 60FPS mode. These two options seemingly apply to Xbox Series X and PS5 according to multiplayer.it, which you'd expect given the similar horsepower available on the two machines.
Now, Konami's upcoming remake is running on Unreal Engine 5, so that may go some way to explaining these figures. UE5 titles seem quite heavy on current-gen consoles, so we're not surprised to see a 1080p figure here in performance mode. Still, we think these are respectable figures given how good the remake looks.
As for Xbox Series S, we're not seeing any specifics for that console just yet regarding MGS3. However, you can probably expect these figures to be a chunk lower on the base Xbox Series system - we just hope some sort of performance modes remains intact for Series S users!