In this case, I believe it would be better to use game time instead of real time for the delay between shots. The coding is also fairly straight-forward. Where you are using two 'int's and one 'boolean' for real-time coding, it only uses one 'int' for game-time coding.
Notice in line 7, we are creating a new missle about every 2 seconds (as long as the 'space' key is pressed).
That is correct. However, no matter the speed that the scenario is set, if you get the delay to a suitable value, then changing the speed of the scenario should not alter the suitability of the rate because we would be using game-time.