Ridley really, REALLY did not want to add back in alien stuff. He made it as dumb and fast-forwarded as possible. The facehuggers impregnate people in seconds and without even having to knock them unconscious. This guy got one on him and it was cut off in seconds, then he bursts later on the ship. The xenomorphs then grow to adult size in another few seconds. Ridley was just like "yeah yeah get on with it, I'm not doing the wait-around thing anymore"