This brings back bad memories, and a reminder it's not always speed, but luck. My son, who some of you will recall, got a Cspine injury on Boogieman 15 years ago, which left him quadriplegic. It was a slow roll a small rolldown, but his wheel slipped off to one side, and he hit headfirst, hyperextending his neck.
We didn't find out about Leatt braces under after that happened, first read about them being used on m/c. I am convinced it would have prevented his injury (and haven't ridden without once since.) And yes, I've crashed a few times since, headfirst into rocks and trees (downhill, and trailbike) ... and think it's prevented injury to me, too.
Everyone I know who knows my son and still rides, uses one of the neck braces.
It's not just about speed, but that likely does increase the risk.
In case you are wondering ... he's doing great, about to start a master's degree in comp sci ... but is still quadriplegic.