It's the case with *all* lenses. TCs will significantly slow down a lens's AF speed. Even still, the Tamron TC + 70-200 combo works very well. AF is fast enough as long as there's no end-to-end focus range hunting.
I have both lenses also and also vouch that the two lenses are of different leagues. There's *much* more to IQ than just sharpness, which would be hard to pick out between the 70-200+TC and the 55-300. DoF control and 3D effects are quite prominent with the Tamron, less so with the 55-300. But at the same FLs and apertures I've not seen dramatic difference in results between the two, except at f/5.6 (f/5.8 for the 55-300), which favours the 70-200.
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