Too fast of a release can interfere with the actual signal waveform and cause distortion or bad modulation effects. The definition of threshold when applied to a limiter or compressor is a signal level below which the processor does nothing. Above a set threshold, a processor performs gain reduction according to its ratio . So, during limiting, an infinite increase in input level is required to produce a 1db in the output level. Well, there are several alternatives available to circuit designers in structuring a detector mechanism.
Peak detectors measure the voltage excursions or in other word the peaks of the polarity.
Peak detection was the backbone of early processors which were designed for broadcast and disc cutting. Unfortunately, the electrical peak value of a music waveform has little to do with how loud the sound is perceived by the human ear, due to the varying waveform complexities. If you were to take two musical waveforms of say a flute and a brass instrument, both would appear to have the same level of audible loudness, but exhibit completely different peak values. If the two waveforms were detected by a conventional fast peak detector, the brass waveform would be read as having a much higher energy level than the flute. If the detector were controlling a limiter, the audible result would be that the complex waveforms, such as brass, etc., would be over limited with respect to the simpler waveform.