I don't know the exact timeline in minutes, but while Floyd was in the police vehicle, he was claiming he couldn't breathe. I think the effects of fentanyl and other substances in his system certainly helped exacerbate his panic issues. It's still no excuse to have a knee on the guy's neck and the main culprit should be punished.
However, because of the highly emotional circumstances of the case, the officers were likely over-charged. Having to prove that the officers "intended" to kill the guy (2nd degree murder) looks to be too high of a bar in this case. I think the DA could likely get a conviction of assault and maybe involuntary manslaughter, but anything more than that will be difficult. I'm no lawyer, but as more and more information is disclosed (tox report, body cams, etc.), it appears that ensuring a conviction will require a lowering/lessening of charges.