Sigma 17-50 f2.8 AF speed
I just got mine today and also noticed that the AF is very quiet, but very slow. It actually seems slightly faster on the K10 than the K5. My 18-55 kit lens is blazing fast by comparison. I haven't had time to really test it much, but it seems to be ok at locking on and there's no obvious FF/BF. Plan to test more as soon as possible.