There are a lot of different kinds of racing kayaks, but almost all have the characteristic of being fast. There are several things that contribute to speed. You will often here about "hull speed" in relation to boat speed. This term is a misnomer in that it implies that there is a speed a hull will go. What it really indicates is the speed at which the hull starts to become rapidly more inefficient.
What hull speed suggests is that longer boats can go faster than shorter boats. This does not mean that they are automatically faster, just that a longer boat starts losing efficiency at a higher speed than a short boat.
The other big factor in speed is "wetted surface area" or how much surface area is in the water. This is important because as you move through the water the water must slide across the surface of the boat. The more surface there is to slide against, the more friction there will be and the slower the boat will go.
The easiest way to reduce the wetted surface area is to make the boat narrower. As a result, there are two primary options for making a racing boat fast: make it long and make it narrow. Therefore, most racing kayaks you see will be as long and narrow as they can get away with. There are typically constraints on these factors. For example, if you make a boat too narrow, it can be so unstable as to be impossible to keep upright. The other big constraint is racing rules.
Most race organizers will set maximum lengths and minimum widths in an effort keep to their events as fair as possible. Their goal is to make the race a test between individual paddlers, not a test of the fastest boat. However, it is the kayak designer's task to try to develop the fastest boat permitted within the rules.
This battle between race organizers and boat designers can create some funny looking boats, but the most common solution creates a boat with a plumb bow and stern. In this way, the boat can get as long a waterline as possible within the prescribed overall length. You will also often see a fair amount of "flare" in the cross sectional shape of the boat. This permits a narrow waterline beam with a wider overall beam that meets the rule specifications.
Neither of the these solution necessarily result in the best boat possible for the purpose if there were no design constraints imposed, but they can make the boat quite fast within those constraints.
The other consideration in a racing boat is stroke mechanics. Despite kayak designers best efforts to make a fast boat, in the end there are really only fast motors. In order to go fast, the person paddling a kayak needs to be strong and have good technique. The design of the boat will often include features that help the motor maintain a good and strong paddle stroke.
The boat should not get in the way of the stroke, and the cockpit should let the paddler move as needed to paddle with full power. This often means the boat is quite narrower in front of the cockpit for a clean start of the stroke. The cockpit may be long to permit the paddlers legs to move.