I have experimented with this lens for quite a while and here on this Tamron 18-270 PZD review, I will be giving my comments on the lens’ speed. By speed I mean the aperture speed of the lens, not the autofocus speed. Since this is not a constant aperture lens, you really must know the aperture speed before purchasing the lens.
It would not be fast at all due to the non constant aperture of f/3.5-6.3. However, if you do not plan to use this lens for dim light situation, this should not be a problem. If you can, try to avoid shooting in the widest aperture of f/3.5 and f/6.3. Stop down by one stop and you will get sharper pictures.
If you want to shoot portrait, set the focal length to 100mm. If you want to shoot landscape, set the focal length to 24mm or below. This is the advantage of this all purpose super zoom lens. You can experiment with different focal length and aperture that suits your needs.