Different swimmers derive different amounts of force from upper and body segments. There is no one number that defines how much propulsion is due to upper vs. lower body movements. While swimmers do get more out of their upper- than lower-body, most still need to use both to have a strong, coordinated, and efficient stroke technique. Swimming fast requires lots of different elements. A fast swimmer almost always has an element of efficiency to their swimming, at least in distances greater than 50-meters. The longer the race, the more that swimmer's stroke efficiency matters in terms of maintaining a fast swimming speed.
Butterfly and breaststroke swimmers seem to have more intermittent force outputs from pulling and kicking, making proper timing of each phase of the stroke vital to achieve high speeds. If there is a pause at the end of the pull phase in breaststroke, the swimmer will slow down until something else (usually the kick) begins. This slowing down and speeding up - deceleration and acceleration - is very inefficient. Imagine you are driving, and you alternately push the gas pedal to the floor then slam on the brakes, over and over again. Not a good way to move!
Freestyle and backstroke usually feature some part of the swimmer always applying force on the water, maintaining some amount of forward movement. Not all swimmers use a steady kick; those that don't rely a bit more on pull timing to keep a more constant velocity.
Another question that often comes up is what is the action or principle that lets swimmers move forward in the water - lift, drag, or some combination? There is still debate on what is the main source of force in swimming, lift or drag. I think drag has more to do with it, with the effects of lift having a lesser contribution to overall fast swimming. The bottom line - practice good technique - or make sure you are using good technique when you practice. Minimize drag and maximize your efficiency to get the most out of both your pull and your kick.