That was my point. What if you don't have 30 seconds?!
This depends on whether you recognize the piece right away, as opposed to a piece you do not, or one that might have possible variations.
I guess the bottom line is that if you are not sure - don't stand there facing someone while you figure it out. Dive or zig zag for cover. Nothing to stop you pointing it their direction while you do and pulling the trigger as you go. It may fire right off, or having it pointed at them might send them looking for cover too.