A change in the speaking rate for the contained text. Legal values are: a non-negative percentage or “x-slow”,
“slow”, “medium”, “fast”, “x-fast”, or “default”. Labels “x-slow” through “x-fast” represent a sequence of
monotonically non-decreasing speaking rates. When the value is a non-negative percentage it acts as a multiplier
of the default rate. For example, a value of 100% means no change in speaking rate, a value of 200% means a
speaking rate twice the default rate, and a value of 50% means a speaking rate of half the default rate.
The default rate for a voice depends on the language and dialect and on the personality of the voice.
The default rate for a voice SHOULD be such that it is experienced as a normal speaking rate for the voice when
reading aloud text. Since voices are processor-specific, the default rate will be as well.