I see a noticeable lag between seeing some adds on the emnity list and being able to select them with TAB. Examples: knight and soldier, dread, gaolers.
Luckily, flash affects them as soon as they're in the emnity list, so I just use Flash+CoS for pickup during that spawn animation.
However I still wonder if it's possible to select them during spawn animation. Maybe enemy cycling works? I'm on PC btw, do PS3/4 players have this issue?