The term "fast ailerons" is an informal term that generally refers to the response of the plane relative to the pilot's input to the aileron controls on the stick or yolk. The faster the plane responds, the faster the ailerons (or other controls).
The roll rate is the speed at which a plane can complete a roll. This depends on a number of factors, including aileron size, maximum aileron deflection, wing length, and speed.
Ailerons that can deflect a lot of air without disrupting the airflow over the wing too much are desirable in an aerobatic plane, so the roll rate can be increased and so the plane will have better roll control at lower speeds. While larger ailerons are desirable on an aerobatic plane, they are also more susceptible to flutter at high speeds, which is not desirable because it tends to rip them off the wing. There are several design techniques that can be used to mitigate this, such as gap seals, counterweights, countersurfaces, etc.