I've been playing UFO AI since around 2.2, so I know that using the flamethrower has usually been associated with a dip in framerate. In 2.4 however, my FPS drops from a steady 47 to 2, and then to less than 1, so much so that it feels like the game is going to freeze every time I use it. A lesser but similar effect is also apparent when firing other guns, I think it has something do with dynamic lighting.
I'm running Windows 7 64bit with an i5-760 and an Nvidia 560 Ti if that helps.
Cheers, Jez.