Even if I just stand still and mount and unmount my framerate drops up to half. If I move the camera back and forth while dismounting it really becomes obvious that something weird is happening!
FPS loss happens during the animation of dismounting. If I use a skill to leave the mount, and skip the animation, there is no fps loss at all. If another player dismounts in front of me there is no FPS loss.
FPS loss is greatly reduced if done in the middle of nowhere with no NPCs or players around.
I have a newer gpu and the latest drivers. I've even cleared out all of eso live folder to download fresh ini files.
All addons cleared and uninstalled. It didn't affect the fps loss during dismount animation.
It doesn't matter if I had riding skill or major gallop passive. I tested on a brand new character.
Race/gender does not influence it. I tested on many characters.
According to google results this anomaly may have started with Elsweyr's release.
Not everyone gets it. I can't figure out why. One guildmate gets it, while another does not. Searching google showed that others get it too.
Thank you for listing everything you've tried so far. Since you're still having framerate issues, please go ahead and open a ticket with customer support.