Attack and release
The two time constants that decide how quickly a compressor clamps down once the signal crosses the threshold, and how quickly it lets go once it drops back.
Both are given in milliseconds, but what the number means varies by manufacturer — often the time to reach about 63 percent of the final gain reduction, a time constant rather than a finish line. A slow attack lets the first few milliseconds of a drum hit through before the compressor reacts, which gives more punch, not less. A fast attack flattens the transient.
Release is trickier. Too slow and the compressor never recovers between hits, leaving everything permanently squashed. Too fast and it starts tracking the bass waveform itself, which you hear as distortion. Somewhere in between lies pumping, a flaw right up until someone builds a dance record around it. Hence auto or program-dependent release, which adapts to the material — the optical compressor got it for free from its photocell.