I just watched that sequence again and I think the serum just started to take effect when he threw the shield into the wall when fighting one of the flag smashers. He did this weird movements with his shoulders like he was getting an adrenaline high. When he grabbed the shield out of the wall, the same flag smasher attacked him again and the next thing you saw him kicked that person threw the door before he bent the iron pipe and the smasher said "Oh Shit".I suppose that’s possible and makes sense on some level, but the thing I’m stuck on is that when he’s fighting at the beginning of the next scene, he doesn’t seem to be powered yet, and then he’s off camera for a while, and then he’s fighting with abilities he didn’t have moments earlier.