Well, it's possible, but the animations are already at 15fps..
In any case, go to line 56 of Sprite_Base and change:
@animation_duration = @animation.frame_max * 4 + 1
Change the 4 to 2, so:
@animation_duration = @animation.frame_max * 2 + 1
Then go to line 36 and change:
if @animation_duration % 4 == 0
change the 4 to 2 there as well, so:
if @animation_duration % 2 == 0
That would change animation speed to 30 FPS.