2D Volumetric Lighting Methods?

Quick note, using GameMaker, doesn't matter though. I've been messing around with lighting for ages now and can never seem to think up an efficient method.

I did in fact accomplish Volumetric Lighting, although it slows down at 70 shadow casters and around 10~ lights. Which isn't entirely optimal, result: http://i.imgur.com/GMfhpNY.png

Now each "shadow" that gives the volumetric lighting effect is drawn pretty poorly. The shadow is drawn at a low alpha, moved back a few pixels, alpha increased, draw, repeat...

The shadow is a sprite and itself could be drawn 50 times... Slow...So I was wondering if anyone had any better, more efficient methods than this.Ideas? All I need is a way to draw the shadows to give a voluemtric effect, afterthat I can do the rest. :)