[Agema]

Well, attacks must cost the 100% AP limit after armour, otherwise heavy infantry (say from base 12 down to 6APs) without quickness would attack every second turn, and they attack every turn. At least as far as I've seen anyway.

I'd imagine 50% quickness could work either as:
1) leaving the armour-adjusted AP cost, just modifying APs.
2) altering the attack AP cost equal to the new quickness-modified AP total
...a) then giving alternate 2 attacks - 1 attack each round.
...b) discounting the attack AP cost (67% in this case) depending on quickness type.

If you're seeing fewer attacks with low AP units, I'd expect you are seeing either (1) or (2a), both being different routes to the same result. This would be because a fast unit could move and afford to attack twice (18APs mean 5APs move and two attacks at 12 each), whereas a slow unit would be more impaired by movement (6APs, only 1 AP free to move for two 4AP attacks.)