The problem with "light" is that it doesn't exist for the computer, it needs to be simulated and that takes processing time.
However, it depends on how effective you want the light to look - as a short rule, the better a light looks the more processing power it needs, but the contrary is also true - if you don't need too fancy lights, it'll reduce the needed CPU power.
The easiest way to simulate light would be to use the opposite - make the room darker outside the light area.
And that can be done with a picture that is black elsewhere and transparent where light should be. Depending on how much light, you'll make some areas with half black and half transparent, and the picture needs to fit the map.
It's a lot of work doing it that way, but it needs almost no processing power.