How will room size effect performance?

I'm planning on making a game similar to Jump King and i have been trying to figure out how to transition from one room to another like jump king does, but then i read that you can just have an infinitely sized room.

so i'm thinking that i would be able to have a camera follow my chararacter, as well as a box slightly bigger then the camera that loads in things that aren't loaded in to reduce lag.

would this be feasible or not. i am fairly new to gamemaker/making games but i don't see a problem why this wouldn't work.

also i should say i'm worried about lag because i don't plan on adding in delta time, i'm making this game for a game design course and don't think i'll end up adding it in.

also also, how do i make it so that when i increase the height of the room it gets bigger upwards and downwards

EDIT:

I've decided i'll just make a 10000x200000 room, that should be big enough and since this is mainly going to be a vertical game if i need to i can just make the player go to the right and fall down and then start going up again.