Remember casting fortify speed 100 points on your horse in Oblivion? They had a 3x stat multiplier, so they actually got 300 speed from it. I actually had 1 white horse that glitched and stayed with boosted speed. Unfortunately when I got off its walking speed was too fast and it walked away at mach 3...
Brilliant. I never paid that much attention to horses in OB, but I'm thinking of firing it up and giving it another go.
As for the OP - as many people have said, it's not broken. I did think the first time I heard a stablehand tell me about how slow the horses were that other stables would have different types of horses. But it makes sense really. A really fast light horse wouldn't particularly do well around Winterhold.