No offense to the two previous posters, but let me try to answer you in less technical terms. Perhaps some will disagree with me here, but this is how I define it. Terms like "fast" and "quick" refer to a component's ability to reproduce the natural dynamics of a sound. Dynamics or transients are the changes from quiet moments in a passage of music to loud moments and back again. Musical instruments when heard live (for example, think of plucked guitar strings, percussion instruments, etc.) have a certain snap to them. There's an immediacy to the individual notes. They sound, well, "live." Recorded music sometimes lacks that natural level of dynamics and--by comparison--can sound like you're listening to the music with a curtain between you and the musician. Instead of a sharp raise of a plucked string going from jet black to instantly there before you, it's as if there is a slow (relatively speaking--we're talking about the speed of sound here) build up of the sound. Good audio systems can come close to capturing that same level of quickness that live music has. So a "fast" or "quick" or "dynamic" component is one that is able to render those soft to loud passages in a realistic manner. At least that's how I've always defined it.
