Speed isn't that great a brand for already fast weapons anyways since there's a cap to how fast they can go. That cap is the reason quick blades can't have the speed brand, by the way, since they're already as fast as weapons allow. Of course, your skills make you attack faster, but that's another issue.
How does this work, exactly? I've used some daggers of speed with spriggans, and they have been just ridiculously fast. Does the weapon speed work more like a penalty to the natural creature speed, thus being more beneficial to already fast creatures?
My previous game where I had one, a spriggan assassin, ended quite recently. In an exceedingly annoying way. I was level 19, on D:20, and found a bunch of orcs and a troll. No sweat, I though, I don't have to retreat into a corridor to fight a few measly orc knights. Well, they took a while to go down, and I decided that, on a second thought, maybe it would be wise to fight them one on one. And while retreating, an orcish priest smote me. And I died. And that was that. I was just loaded with stuff that would have saved me if only I'd thought of using it!
This is a depressingly common trend in those of my games that last beyond the first few levels. The first few levels, I just storm through the dungeon, with little regard for personal safety. On CDO, my best score is 202 809 (the spriggan assassin I just mentioned, by the way) and my average score is 1571. I think the contrast was even higher when I played on CAO.