If using a Siri voice for VoiceOver, it is not possible to change its speech rate

If you select a Siri voice to be used as the VoiceOver TTS voice, its speech rate does not change to reflect any change that you make to the speech rate setting in the VoiceOver Utility or changed via the speech attributes settings accessible through a keyboard shortcut.

1. When VoiceOver is on, open VoiceOver Utility by pressing VO-F8.
2. Click Speech in the category table, and then click Voices.
3. In the “Customised language list table”, select a Siri voice from the “Voice” dropdown menu (if none are currently downloaded/available, select the “Customise” option from the menu and then enable one or more Siri voices).
4. Check that VoiceOver is using the selected Siri voice.
5. In the “Customised language list table”, change the “Rate” value.

You should find that the speech rate of VoiceOver does not change to reflect the new “Rate” value.

The same behaviour occurs if rather than VoiceOver Utility you use the keyboard shortcut for accessing/setting speech attributes (VO+Command+Shift+Arrow Keys).