I guess this time is not long just to allow you to make coffee (I may be wrong, UFO AI devs might want to make fun of players ^^ ).
More seriously, if there was an easy way to speed up the alien turns, I guess it would already been done. I don't know what slows down the turn (computing for aliens within vision ? Decide what to do ? Pathfinding ? ). But AFAIK, there is no way to disable anything that increases alien turn speed. And during alien base attacks, where there are loads of them, it can take ages as you already noticed :-(
Anyway, I remember seeing some threads about speeding up alien turns so it seems to be already known and is probably not easy to deal with. If you have some skills with code, you can have a look