I'll tell you what I've noticed when playing killer on the Xbox One X.
The frame rate increases with each survivor eliminated from the game, when all 4 are dead the frame rate is compareable to 60fps, try standing near a wall or the killer shack as the last survivor dies on the hook and pan the camera around.
I'd "guess" this is down to CPU where it has to do less and less work with each player character removed from the game.
The X has a meaty GPU but it's CPU is lacking which would make sense with the above and your post regarding the tutorial which one has 1 player, you.

Dwight_Confusion said:

The the FPs was improved during the tutorial?

Not quite understanding what you said. Wouldn't it be better anyways in that setting? Smaller area? less people etc?