Speed is how fast you go once you reach maximum velocity. This is not how quickly you reach top speed (that is acceleration), but the actual speed that is theoretically availiable. If a course is full of twists and turns, top speed may be irrelevant. If you want it to go faster in twisty situations, you need to use a kart's Mini-Turbo, or items such as a mushroom, triple mushroom, mega-mushroom, or star.
Small-sized characters have usually have low speed, but a high acceleration.
Medium-sized characters often have fairly equal speed and acceleration. A midground of the other two sizes.
Large-sized characters commonly have high speed, but a low acceleration.
