Holiday, I actually found that capx here on the forum.
Thanks, R0J0hound !
You example does work a bit faster. I have an old Android mobile phone that I use for performance testing and it's showing 45-50 fps. (about 10 fps more than with particles)
Unfortunately, the Subtract effect is still quite slow on mobile. When I add two big gradients like that into my game, fps drops from 50-60 to 20-30...
So I think I'll have to use particles, but lower quality.
If I make the image smaller, significantly reduce the rate and fade out time and move them closer to the borders of the screen, the fog looks good enough and only costs 2-3 fps.