said by Pentaxian 
:It seems all of these mega-zoom lenses whether SDM/HSM etc or screw-drive are all fairly slow. I haven't seen one from nikon, Canon, Pentax, or any third party mega-zoom like this that is fast - Does Nikon even make anything with that much throw?
No but on the Nikon's if you compare the same lens like the Tamron 17-50 2.8 VS the new version of the Tamron 17-50 2.8 with the internal motor, the old screw drive versions focuses faster. That's a fact. In fact I wished I had purchase one of the old version before it was replace by the one with the AF motors. 
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