I find that I can run perfectly well with fog, filtering, coronas, insanely high resolutions, etc. in OpenGL mode, no slowdowns whatsoever, even if the turret bullets have coronas attached to them and there's a room full of twenty seven turrets all firing at me at the same time.
However, I get horrible lag in software mode, even at 640 x 400 resolution in a small map like GFZ1... I mean... 27 fps on average... In GFZ1?... Wtf?
By the way, I prefer 2x coronas only because it's hard to see the ring sparkle glow otherwise and stuff like that. :P Also, the coronas on the emeralds placed in maps is insanely huge either way. x_x Like a street light, except colorful... Except for the pink emerald, which is white for some reason. And speaking of, a black corona might look really cool for the black Chaos Emerald, even though it seems like an oxymoron...