There is: setgamespeed(float). It defaults to 1, so setgamespeed(2) is twice standard. You can also call getgamespeed() if you forget what you've set it to. setgamespeed(getgamespeed()*2) will always double it.
For those who don't know, F2 can be used to open or close the console window, where you can enter commands in Rome.