Things feels different once game speed exceeds 2f. I would prefer to set game speed to 2.5f or even 3f, but sounds do not always play at the expected time and at 3f or higher, animations like venting simply do not look right.
On the other hand, I cannot tolerate playing at the default of 1f - it is far too slow. 2f feels much better, but even then, much of the game is still a bit slow-paced for my liking (aside from Hyperion vs. Hyperion or Afflictor duels) even at double speed.
On an somewhat unrelated note, Endless Sky was recently updated to 0.9.7, and added a Caps Lock toggle for 3x speed (mainly to speed up travel). The game is much more fun at triple speed, although late in the game after a build-up a giant fleet, the slowdown from so many objects simply offsets the time shift. Needless to say, I set the game at triple speed and leave it on at nearly all times.