Sigma 17-50 f2.8 AF speed
I haven't experienced slow focus on my K-5. That said, I've not been especially impressed with its IQ relative to the 18-55 kit. If I wasn't so lazy, I'd use my 35/2.4 for low light, my kit otherwise and sell my Sig. I do like my Sig 10-20, just not my 17-50 so much.
Have you tried to test the AF fine adjustment to make sure the focus is right on?
Mine is great in IQ.