In general, my experience is lag free (though I do have a problem where pathing NPCs show up where they'll end up before they get there, I don't think that's actually lag). I do have loading freezeups, however. The guy in the video had lag as well as a system problem. I am no expert in diagnosing the source of slowdown. I'd say if your HD light is blinking contantly while you're playing, you could benefit from more RAM. Otherwise, your issue is elsewhere.

My issue is clearly with viewing areas that are not precached, mostly when I quickly change camera direction.