I had watched MKBHD's video, went back to look at that and he says "maybe 5 seconds" but in the video he's doing a voiceover on it seems like it's maybe 2 to 2.5 seconds to go from the start of the transition to the screen saying "transition complete", and then another couple seconds after that for the dash to then do whatever switch it needed to do. His video doesn't have any audio though at that point so you can't really hear when the engine turns on, I'd expect it when the screen says transition complete but I don't know for sure.
Anyway, sort of splitting hairs, but 2 seconds seems ok, 5 seconds seems like a long time!
The old videos of the the Toyota TS030 firing up it's gas engine I think are my gold standard of electric to gas operation!