You can most definitely pass a slower horse

.Just do it smoothly ,maintaining cadence,just a subtle moving out enough to pass & then back to take your place on the rail, it is just a continuing on along just as there was never any horse there in your way.If done properly it is not frowned upon,but can actually show the control & consistent smooth way of going of your horse that judge will take notice of

,the same way he can take a negative notice of if passing is done in sloppy manner or if you try maneuvers to fit in behind slower moving horses

.