Things like fixed props and squared-off sterns can slow a boat considerably. Wider displacement hulls don't move as easily through the water as narrower boats. Bow thrusters add turbulance. Turbulance is drag. It's all pretty much common sense.

If we let our folding prop free-wheel I notice the reduction in drag when I lock it... we pick up about half a knot boat speed (and as you can see in the image, a Volvo-Penta 3-blade folding prop is not the sleekest when folded, compared with some other designs).