If anyone else gets stuck in the “slow bug” where your character slows down to a crawl completely out of nowhere, all you have to do to temporarily fix it is hold heavy attack. You may also need to drain your stamina to zero but I think that was an unnecessary step.
Regardless, if you are going at a snails pace even on your fully leveled mount with rapids, just hold heavy attack. The game will catch you up right back to normal speed.
A special thank you to the awesome person who told this to me last night. You’re a hero.
Note - this may only be a Cyrodiil bug as it’s only happened to me there. Thanks!