Here's some professional info for you:
There are a few parts to rendering a scene. One, the camera that the player views the world from. That camera also forms the basis of determining what gets rendered by the hardware. Two, it is attached to a viewport, the surface that the scene will be rendered to based on what the camera is viewing.
To do split screen, you need as many viewports as you have players. That means a camera for each person, too. That means, rendering multiple times per viewport you have, each with their own bits of geometry being viewed. With one camera, you can see at most a fourth of the geometry in the section of level you are at. With more than one person, more of that area has to be rendered.
This is all just basic rendering. To do lighting, shading, and other special effects requires even more effort multipled by the number of extra screens you want. Split screen requires that you aren't maxing out the hardware to start with, otherwise it will destroy your preformance.