Another big major factor is images, not all business have a professional photographer to take the type of images needed for a great web design. This is something you will need to work out with the developer if stock images could be mixed in or if they can take those pictures and do that branding for you. Lastly another large factor in time we see is stakeholders. The more cooks in the kitchen the longer it takes to get concurrence. While design by committee sounds like a good plan really you need a point person who can make final decisions for your business.
We hope this helps and gives you a few points of discussion when working on your next project timeline.