Attack Time
How long the compressor takes to control the signal after the actual overload occurs. In a good compressor with adjustable attack time, expect a range of adjustment from 500 microseconds (us) to 100 milliseconds (ms). Some manufacturers specify this time from the beginning of overload to some arbitrary gain reduction -- perhaps -3 dB. Others specify it from the start of overload to the time the gain stops changing. Perhaps the most meaningful is to specify from start of overload to when the system gain is within, say, 3 dB of the final control point for 10 dB of gain reduction. Unfortunately, in the war of specsmanship, this measurement suffers by comparison. In a system where this parameter is adjustable, ridiculously short times are often spec'd just to look good. Some manufacturers state the attack time as from onset of a very large, very fast overload to the beginning of gain control. This is an artificially short time since it really doesn't reflect how long it takes to get the overload under control. The important consideration is "how long does it take to control an overload?" Equally important as speed, is the shaping of the attack function. If badly done, even a slow attack will sound abrupt and "clicky." Unfortunately, the buyer is not usually informed as to this critical part of the design. This is where a reviewer's article could pay off, as well as a carefully done listening test by the potential buyer. If during testing, the compressor sounds intrusive when the attack is reduced below 1 ms, try a different compressor.
Release Time
How long the compressor takes to relinquish control once the overload passes. Same problems of specification as the attack time, but of less consequence. Normal adjustment range is from 100 ms to 3 sec or more. A short release time of 100 to 500 ms is a good starting range for spoken voice, while the longer times are better for instrumental music. This time inversely affects the distortion added by the compression process, as will be shown. Release time is usually adjustable. Here again, the buyer is at the mercy of the designer. Much has been done with release circuitry over the years to produce good compressors. Terms like "program dependent" and "dynamics dependent" abound. Some have genuine meaning, while some are hype to get the buyer's attention. Bottom line? How does it sound when compressing?