a "fast" amp is one that has the ability to output "naturally" a quickly rising transient input from recorded sources like piano. while the "quickness" of an amp is affected by characteristics that include bandwidth, the "objective" measurement of "speed" is generally expressed by slew rate (theoretically, the higher the expressed value, the faster the amp). as with damn near everything in audio, tho, your ears are the ultimate measuring device. once you understand what "fast" means, you'll know it when you hear it.
