In short, I have not noticed a running speed problem in Arx. Try raising your Dex/Agility stat and see if that helps.
You can't get speed until mid-game or something where you find the Movis rune, and it costs quite some mana. I'd say you can cast it reasonably often, but don't expect to be chucking lots of other spells at the same time.
There's a cheat spell using easter egg runes that permanently speeds up AS to a tolerable level. In spell-casting mode draw these three runes (attached): R A F
Am Sheagar will now move at a reasonable RPG clip.
For the full set of cheat runes go here: http://www.uhs-hints.com/uhsweb/hints/arxfatal/339.php