When our light source is a plane rather than points, is there any way that computes faster than "rasterizing" the light source?

^This is an interesting question, but I can't think of any reason why a planar source would be fundamentally easier than a point one. You still need to understand which object the light source hits first.

That said, a planar source of light that produces parallel/collimated beams might be easier to render for than one that produces more diffuse rays?

A light bulb emits light in all directions, I wonder how to model/simulate this?