Huge lag when smoke / semi-transparent / alpha effect

[eddiefur]

I don't understand why games lag so much when there is too much semi-transparent polygons on the screen. My computer run at max speed every games in a normal scene, but when semi-transparent polygons start to take a lot of place on the screen, games start to lag ridiculously. In my head, polygons are polygons and textures are textures, it's all the same, but I see that it's not the case. Is there a way to fix or disable those semi-tranparent alpha laguish polygons?

Thanks!