He never moves fast enough.
He's slow before the serum, but at least then he has excuses. His lungs, his bones, the way his body never quite lives up to the actions he wants to make — the connection between brain and muscle is off and he just can't force it to work properly.
That was why he liked to draw, he supposed. The movements of his hand were small enough that they didn't require effort, his arm and fingers responded the way they were supposed to, and even after the serum, after a few clumsy efforts at drawing where he broke the pencil (it took a while to get used to the enhanced strength) it was a relief to know that he still had that outlet.
The serum doesn't stop him from being too slow, though. In New York, after the portal is closed and the adrenalin has worn off and the bone-tiredness has set in (not fatigue, exactly, but just a crushing need for stillness and quiet) In the ruins of blasted cars and falling debris there are bodies, bodies that he couldn't save, and he blinks, remembering other bodies, older ones in uniform, people that he killed, not just people who were killed in front of him.
"Steve Rogers, are you all right?"
He blinks. "Uh. Yeah, Thor. Just… there's a lot of work to be done. Lot of homes are gone… so many…" He trails off, seeing the uncharacteristically grim look on Thor's face, remembering that it was his brother who was behind this. He shakes his head. "I guess I'm just tired, that's all."
"Here," Thor says, helping him lift aside a fallen piece of masonry.
They work in tandem for a time, the rhythm and movement similar to how it had been during the fight, a silent acknowledgement of each person's strength and determination to make up for things they thought were their responsibility.
Moving forward.