Asks, High speed on a bicycle on a descent on my experience is never related to gearing-- certainly not with panniers. Every time I've hit 80,90k I'm way past spinning out anyway.

As with going very fast on a motorcycle, it's all about "the right place and time"---experience, reactions, judgement and bike handling abilities all come into play.

And one persons crazy fast light speed, is another person's calm and measured speed-- but again, time and place, and never putting others at risk.