I've just spotted this interesting article about the fastest ascents and descents in fell and mountain running (in the world!). The author Anthony Kay says...

"I have concluded that the fastest sustained descent rate ever recorded in any mountain race is 1.365m/s, achieved by Keith Anderson in the 1990 Pen-y-Fan Race, when he descended the 580 vertical meters (in 2,450 meters horizontally) from the summit of Pen-y-Fan to the finish at Cwm Llwch in 7:05. Just to show that this wasn’t a fluke, he did the same descent just 5 seconds slower in 1992. No one else has come anywhere close to this descent rate in any race in which descent times are recorded, but, of course, faster descents may have been achieved but not recorded at other races."

Anthony goes on to suggest Pen y Fan has the perfect gradient for a fast descent. However anyone who has done the race will know this is divided into a hellish steep first section, then a gentler section across the boggy bit, then a gradient in between, on fairly smooth grass. For me the decider is the ability to get down the upper steep bit fast - the gradient here is too steep for a running action and so technique is everything; many very good runners are reduced to a tentative totter.  Keith Anderson was apparently the ultime descender, and it must have been an almost unique ability to tackle this first section that puts his two descent times more than half a minute faster than anyone else. The next fastest was Colin Donelly; in 1990 he was 36 seconds ahead of Anderson at the summit, but must have been gutted to finish 2 seconds behind with a descent of 7:43.

For the ladies, the only recorded descent under 10 minutes was Tricia Calder, who did 9:15 while setting the 38:15 record in 1990 - it must have been a good year.