I personally enjoy shooting every last alien. Even if I've done it twenty times before, I'm not in any rush to go back to orbit. The grind for loot and bounties are unimportant to me if I'm not having fun. I always appreciate when players stop rushing ahead and play the game with me (and I try to display that appreciation with waves and dancing). If not that's fine too, I usually don't take too long and catch up to where ever they are.