Video your run and watch your stides.....each stride is 0.1th of a second. You should only make 3 to 4 strides from your pocket, around the barrel and out. You should be rating and reaching for the horn at 3 strides out from the pocket.....sooner than that is costing you time.
Be sure you are hustling once you clear the barrel......I have to watch myself looking for the next barrel too soon and not finishing the barrel I am on.
Do about 4 miles of exercises before you start your "training" build up his strength and stamina and lungs. What is your method of teaching the barrels, if you can explain how you do this, we might be able to identify something that might be throwing you or the horse off the slightest (in no offense in any way) Also, I trotting up and down a hill (specially up) will give him lots of long muscles so he can make his bigger strides.