A roof rack can have alot of considerations for driving. Firstly lets look at the wind tunnel. When you put a large kayak on your roof you are increasing the size and mass of your vehicle wind will now be hitting you on a larger space then it would have been with your car. Think for example of a parachute. When someone lets the parachute go they come to the ground slower due to its mass. This is the same with large objects on your roof it slows the vehicle down.
Additionally to the wind tunnel effect there is the extra mass on your vehicle causing it to be slow due to its larger weight and also causing a loss of gas mileage.
Finally the instability that may come with extra mass and weight could effect turning slightly.