You have to change your movement speed in your mounts tab. Under movement you probably have "Common tack". Click "Change" and choose "Exceptional tack". You can actually mix mounts' speed and appearance, you can even make a regular horse run at 140% speed if you own orange mount.
None of these solutions worked for me. Devs really should take a look on this problem. I doubt that's something complicated because before mod 5 everything was fine.