I own six horses irl and I have been riding for twenty years. I've competed on horseback, I've made a living on horseback. All suggestions that horses should be more sluggish or response have obviously never ridden a tuned horse.
http://www.youtube.com/watch?v=ZvuSvwb8DPg&feature=fvwrel Fuck the music, mute it and watch. That's how agile a horse can be. The horse is motherfucking side stepping. I'd like to see that shit. Diagonal movement.
http://www.youtube.com/watch?v=ZvuSvwb8DPg&feature=fvwrel No hands. Sudden stops can be done on horseback, more or less.
Personally, I think the idea that a slight slope is going to radically slow down a horse is also ridiculous. Just my couple cents.