There are several threads here and on Steam forums with every player stating that they have this.
The stuttering occurs when the animation restarts its loop. The whole image just stops as the animation is about to restart, when we are on horse walking speed. It doesnt happen on the other horse's speeds.
Please fix this, it's like this from the start and the devs haven't acknowledged this bug...
I always avoid the horse walking speed because of this.
CDprojekt, you need to at least acknowledge that there is an issue here and tell us your working on it. It's not a GPU problem or a problem with anyone's PC so it needs to be patched ASAP. It's been reported for over a month now and yet still no word, bit disappointing that you seem not to care enough to do anything about it. FIX IT.