I keep old Lurker lists, as a 'stable' of handy threat to throw about when needed. Lists at different strengths that can be reworked at a moments notice so minis can be thrown down pronto.
Hardly calculation, and can lead to some repetition, but speedy.
Mr P: That is a great idea.
I have something very similar in operation with the Cultists, which form the mainstay opposition at the beginning of most of my players games (I am usually a GM for the players, playing Lurkers on 95% of occasions), where I have a base 15 BP that I just add nastiness to as required.
I have several different Lurker groups, Cult, Asylum, Wildlings, Scarecrows, Rogue Agents etc where this works.
It is always good fun to try and make some really nasty stuff for a change, though, as I am usually quite nice to the players early on.