I've set the bits_per_month to 18 and fast_forward to 50 (the default) in simuconf.tab. In theory, a month should only last 5.243 second in fast forward. However, I measured that it actually lasted about 12 seconds, which is longer. I haven't built anything yet on the map, and it doesn't seem something related to the performance. So the only possibility is that the fast_forward value is not exactly how many times it gets faster, right?
The fast forward gets as fast as it can while trying to stay close to the intended factor. If a map is empty, the load is too low so it is stuck between too fast and too slow for 1 ms waiting increments. When the map is larger or more to move on it, then it first gets more accurate until at a certain point it cannot keep up anymore.
You can see the current time acceleration in the lower right corner.