It is an incredibly fast response, though I would imagine that when flying at a low level like that, one would be primed to eject at the first sign of any anomaly.
Not sure what you mean by “activation.” Turning a radio on is very simple and the video doesn’t show the pilot’s hands anyway.
I make it to be about 10 seconds from ejection to landing. That’s the same as the second test in this video: https://www.youtube.com/watch?v=Z1cnvJO1TF8 (the first one was shorter but didn’t appear to be survivable…)