I posted in the wrong section, should have been in general, damnit.

Ajax_22 is correct concerning that player obviously using a cheat engine. But I disagree about the fastest runner in the game... Orc's and Bosmers would let you build to it easier, and both Sorcerers and Nightblades have class skills that can help make it easy as well, but it's still super easy to hit the speed cap with any class/race.

Unmounted movement speed is capped at 200 (or +100% base, depending on how you choose to look at it). I forget the cap for mounted movement speed but know it can go above 200. More specifically, an untrained horse would start below 200 but a well trained horse would be above.

I've got a couple of speedster-spec'd toons and I've got to say they've been really sad since they put in the speed cap because it's really easy to hit that cap even when only using a few of the many speedy options available. One of mine can even stay in stealth forever going top speed. There are some sets to help with sustain, which could give the appearance to others that you had infinite run sustain but that's just because you can get the resource cost down to nothing if you build right. And while it's very easy (and fun) to keep up with a new player's untrained mount, it's simply impossible to keep up with a well-trained one due to the unmounted speed limit.

So there's just no way they were faster than you, if you were going max speed mounted, without them using a cheat engine. If you see it again, do your best to report them in-game.