to the game developers out there: what is the issue with split screen?
For every player viewport (aka "screen"), you're rendering the game another time from an additional camera's point of view.
In 2-player that's 2x the stuff being rendered, in 4-player that's 4x, etc... this tanks framerate unless handled well. Some tricks to make sure this doesn't become a problem on a weaker machine are to lower the resolution of each player's scene, force the game into lower graphics quality settings, and even cap the FPS to a lower number while in splitscreen - maybe even a combination of all 3.
Depending on the level of freedom the game implements, each player could also move very far away from the other player(s) resulting in more of the game's level being required to exist in RAM at one time. If the game is optimized so that, for example, only assets within 50 meters of a player need to be loaded at any given moment, you've got a problem when player 2 jumps into the game and runs 50 meters or more away from player 1. All of the assets around player 2 that player 1 doesn't need, now need to be loaded
in addition to everything required for player 1, and it only gets worse with each additional player. If not dealt with, this can lead to maxing out your RAM budget and either tanking performance further or just not being able to run at all.
There are optimization tricks for the RAM issue, such as forcing the game into super-low quality asset mode (models, animations, textures, audio files, etc.) during split-screen so the same assets won't fill up your available RAM completely, or just simply don't allow players to wander past a certain distance from each other.
On top of that you've got the increased CPU cost incurred by each player, depending on the complexity of the game logic and physics going on around each one.
It's entirely solvable on any machine if you care about it enough; most games with split screen modes are making sacrifices somewhere in either the visual/audio fidelity of the game or the game design itself and we've still had fun with them for all these years.