Lately I've been getting two different speed related bugs that can only be cured by logging off and relogging.
These are not new, but the reliability that they happen is. I'm sure this is more fallout from lag.
First, there is the very slow bug. Your toon will not move faster than a snail, everything is is ultra slow motion, but just for the toon having the bug.
Second is a no sprint bug. No matter what you do, mounted or dismounted, you can not sprint.
The only way to get these bugs off you is to relog.