As far as I know, in any vehicle, there is a direct relationship between speed and severity, statistically speaking, with "lots" of studies showing that. This also applies to being hit, or front-to-front crashes. There's even the term "high-energy-trauma", applied by health professionals to injuries involving high energies, for example high kinetic energies.
Of course there are confounding factors: are fast riders (professional riders?) more skilled? Are fast racing events the place for a more "focused" riding? Do they fall less, or are someow more prepared to fall?
Anyway, I cannot see how a SLOWER fall could hurt MORE than a fast one. It's a matter of kinetic energy, speed of response of human motor coordination, even common sense.
Although I have not evidence to bring about, I hope this helps someway.