Killing someone who is unconscious instantly now consumes a combat round. This is the same as many other actions which you are used to not being able to spam, like eating and drinking. Out of combat, it will stop you from doing another action that requires a combat round. Inside combat, you will skip your next attack and do nothing.
Please plan your next murdering spree accordingly, as you will not find it so easy to pick off 5 gangers in as many seconds.