i've tried optimizing this shader with squared distances and so and i haven't got much better results, so i'm going to change all this shader-based lighting for something similar to what kalle_h said, but have to take a look at the additive blending and see if it's what i'm looking for.
Then what is the alternative?
I thought that it would avoid redundant computations to have one texture on top of the other and light every pixel of them.
I've also though that vertex lighting would not be easy to apply as a big texture is represented by a simple quad...
Sorry about the bad spelling, i've corrected every mistake that i've found.