Some people reporting near-death experiences say they felt terror in the presence of unimaginable evil, not panic, and there was nothing lethargic about it.
You’re referring to the speed of hallucinations. I’m not sure how to measure that. But, when the heart has stopped, the body has stopped moving entirely. What I’m referring to is the early stages of oxygen reduced brain function, still conscious but impaired; the body still functioning IN a lethargic condition.
You can probably do a search on USAF studies on oxygen deprevation and it’s side affects.