Voice Guidance speed

[AuraDMG]

I've just got a Sky Stream box and was happy to see they have a lot of accessibility features that are really useful, especially the screen reader. It turns on and off and functions fine, but the speech is a bit slow. I can see there's a setting in the accessibility menu to change the speed but there seems to be no difference between Slow and Fastest. Is the change supposed to be immediate, or is there a problem with it?