speed factor[c]
factor is
either the ratio of the new speed to the old speed: greater than
1 speeds up, less than 1 slows down, or, if appended with the
letter c, the number of cents (i.e. 100ths of a semitone) by
which the pitch (and tempo) should be adjusted: greater than 0
increases, less than 0 decreases.
Technically, the speed effect only changes the sample rate information, leaving the samples themselves untouched. The rate effect is invoked automatically to resample to the output sample rate, using its default quality/speed. For higher quality or higher speed resampling, in addition to the speed effect, specify the rate effect with the desired quality option.
See also the bend, pitch, and tempo effects.