Horse Speed does not remain constant

I was playing on snapshot 13w19a and I found a fast horse to use for a map. I balanced the map around that horses speed and jump height. When I logged in with snapshot 13w21a, the horse was much slower, as were some other horses I had tamed. I tamed more horses and eventually found a fast horse that I could use, but after I dismounted him, the next time I rode him, he was slower. This was not a small change either. The horse was very fast, and now as slow as they come. This inconsistency makes it very hard for me to continue with my map, and I don't think other players will like it either.