First page of Chapter 379.
Lady Nagant was still in Central Hospital after Kurogiri was activated. Note: The doctor states Nagant walking could rip open her wounds. She can barely walk.
Kurogiri was activated by Chapter 374 and was shown at the Coffin in the Sky in that same chapter.
Chapter 377 picks up right where that scene leaves off. It even uses the same art from Chapter 374 to show when Kurogiri arrived.
Shigaraki gains his Quirks back mere seconds after Kurogiri arrives and he destroys the barrier around the Coffin in the Sky. Lady Nagant is still at the hospital at this point, that's fact. The timeframe is for her to get to a building's roof is Chapter 377 to the end of Chapter 378, as the latter is when Lady Nagant shoots Tomura.
That feat isn't as impressive as you'd think.
For example: Let's say AFO was 200 meters away from Bakugo when the latter started to move. Let's also say AFO has a speed of 200 m/s. I'm pulling a random speed out because it doesn't matter which speed you use, the percentile will be the same.
Using that timeframe, Bakugo takes 1.5 seconds or 36 frames to reach AFO's position. AFO, with a speed of 200 m/s, would've traveled an extra 300 meters in that timeframe.
200+300 = 500 meters would be the distance Bakugo would have to move.
500/1.5 = 333.33 m/s would be Bakugo's speed. Which isn't even 2x faster than what AFO is moving.
Increasing the distance would make the difference in speed greater, but the results aren't going to be good from what I've checked.
AFO wasn't looking at Bakugo, so you cannot say he covered that distance before AFO could react. AFO only noticed Bakugo when he came into his field of view. AFO's eyes turn to look at Bakugo right after he shows up. We cannot say if Bakugo could've blitzed him or not.
That's the reason why we cannot use a reaction timeframe. AFO would've had to be staring at Bakugo's position and be unable to react for that to count. The same was true in the manga, which is why it wasn't calculated. AFO turns around and only notices Bakugo when he puts his hand on his face, at least that's what we see.
It's also not that hard to find where Bakugo is. AFO and Bakugo are flying in the sky, so they're moving in a straight line. You can also calculate AFO's speed in this moment and it won't be calc stacking because it's all the same moment. Due to cinematic timing and the flashbacks, it's impossible to say how far AFO travels, so the distance would be low balled. However, the results weren't very good.