When submitting a bug report, please use the below template and thoroughly fill out the necessary information.

Date & Time: Any time
Frequency: Reliable
World name: Any
Character name: Any
Retainer name: N/A
NPC name: N/A
Monster name: N/A
Class/Level: N/A, but you must have the increased mount speed for that area
Party or solo: N/A
In-game time: N/A
Area and coordinates: On land and underwater (sometimes flying, if you jump at just the right time)
Housing: N/A

Steps:
1. Get on a mount and press autorun (or move forward) immediately as the mounting animation completes
2. You will occasionally be moving very slowly instead of the fast mount speed
3. If you stop for a moment and press forward again, your mount will then be moving at the increased mount speed

I believe this is related to having the increased mount speed. The game seems to not realize for a moment after you've mounted that it should be moving at the increased speed and instead moves at the slower speed (or possibly even slower than that, as it's hard to gauge).