Quite a few of the 2D games that I have seen use per-tile lighting, where each tile has a set brightness (continuous scale of 0 to 1, for example).
One way of calculating the brightness of any one tile is to loop with all light sources and add a brightness value to the tile based on distance from the light source. However, with 100^2 tiles and 100 lights, that's a million iterations.
What's a CPU efficient method of calculating the light on each tile with multiple light sources?
Light sources that have objects in the way of tiles should also have a lesser impact but it doesn't matter if this doesn't resemble shadows.
The light sources change every few seconds.