It would halve their effectiveness in your example, not quarter it -- 4x attack power, 2x speed = 2x difference in number of kills it makes. The balance of Fast & Dangerous is definitely a bit different than the balance of the Normal combat style for this reason, in more places than just this.
For example, invading enemy ships have the advantage over you if you are quickly trying to build a fleet out of your docks (since your dock construction is no faster, but they are able to close the distance twice as fast and then do 4x as much damage in the same amount of time once they are there.
These modes are not meant to be equivalent (they could not be, since only the combat is sped up), but rather are just two broadly different ways to enjoy the game. I prefer Fast & Dangerous, but I think there are as many drawbacks for me as a player as there are for the AIs with things like Ion cannons, so in practice I think it works out around the same overall. However, a cunning player who wanted to game the system and quickly switch back and forth between the two modes could exert an unusual advantage, and that's why I've made it so that you have to save and reload the game between each switch.
Speaking more to your ion cannon example in particular, in practice I think even that makes little difference, since usually there are ships guarding the cannon, or other things in the way that make it take you longer to get over there, and in Fast & Dangerous or the regular mode the ion cannon has the chance to pretty much decimate anything that is low-level enough to fall under its attack.
