From the Auto Tune manual (
PDF)
Retune Speed
Auto-Tune Evo also gives you control over how
rapidly, in time, the pitch adjustment is made
toward the scale tone. This is set with the
Retune Speed control (see Chapter 3 for more
details).
Fast Speed settings are appropriate for short
duration notes and for mechanical instruments,
like oboe or clarinet, whose pitch typically
changes almost instantly. A fast enough setting
will also minimize or completely remove a
vibrato, as well as produce the infamous
“Cher
effect.”
Slow Speed settings, on the other hand, are
appropriate for longer notes where you want
expressive pitch gestures (like vibrato) to
come through at the output and for vocal and
instrumental styles that are typified by gradual
slides (portamento) between pitches. An
appropriately selected slow setting can leave
expressive gestures intact while moving the
average pitch to the correct tonal center.