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the AF-speed of the Tamron 28-105/2.8 is on the slow side for sure, but I've seen worse AF-performance.
I'm using this lens from time to time because it's focal-length-range is so nice (exactly the same goes for my old copy of the Sigma 120-300/2.8).
It's not a great lens, only it's physical size is impressive. If you can find a mint example for a really good price, you could give it a try.
Here are two shots, taken a while ago at f/3.5 on a D700 at ISO3200:
Tamron 28-105/2.8 at f/3.5, D700, ISO3200, 1/500sec, cropped a bit, with some sharpening good enough for a fair sized news-paper-print
a ghosting-check ;-) same settings as above
Please excuse my weak English, Georg.