To make them faster, have a decent restoration skill and have the ability to make your own spells.
Make a spell that fortifies speed, but make it touch instead of self (you can do target but if its just to your horse, that's wasting magicka).
I made two spells that added up to be 70 speed and it was hilarous how fast my horse was.
I recommend 20 to start with, with the longest duration period you can get.

WARNING: it's really easy to kill your horse in turbo mode because when you run over cliffs on your horse it gets hurt. That's why I recommend a small number. And since it's really easy to hurt your horse, it's a good idea to get a restore health spell to heal it. You can usually find those spells at churches.