I've been wanting to test this on various mounts. Here are my results:
Rider - Wood Elf. Mount skills maxed. Wind running passive active (ie: 120CP in Lover) No other speed buffs except as detailed below.
Test track is from Honrich Tower Wayshrine to Riften Wayshrine along the main road. Same route each time. Rapids reapplied at 1 sec duration remaining.
All times in seconds.Doom Wolf Mount
Without Rapids - 102s
With Rapids - 80s
Sorrel Horse
Without Rapids - 102s
With Rapids - 80s
Dwarven Spider
Without Rapids - 103s
With Rapids - 80s
Green Narsis Guar
Without Rapids - 102s
With Rapids - 80s
Hammerfell Camel.
Without Rapids - 103s
With Rapids - 80s
Nix-Ox War Steed
Without Rapids - 103s
With Rapids - 80s
Leopard Senche
Without Rapids - 103s
With Rapids - 80s
(Margin for error +/- 1s)
Conclusion:
All mounts have the same speed.