https://github.com/WebKit/WebKit/blob/fca03d0dccb29ce539876bba8d7180f93ecc12b9/Source/WebCore/platform/cocoa/PlatformSpeechSynthesizerCocoa.mm#L96:

    // WebSpeech says to go from .1 -> 10 (default 1)
    // AVSpeechSynthesizer asks for 0 -> 1 (default. 5)
    if (rate < 1)
        rate *= AVSpeechUtteranceDefaultSpeechRate;
    else
        rate = AVSpeechUtteranceDefaultSpeechRate + ((rate - 1) * (AVSpeechUtteranceMaximumSpeechRate - AVSpeechUtteranceDefaultSpeechRate));

This maps the slower rates linearly from AVSpeechUtteranceMinimumSpeechRate to AVSpeechUtteranceDefaultSpeechRate. However, while this maps

1 → AVSpeechUtteranceDefaultSpeechRate

it maps

10 → 0.5 + ((10-1)*.5) = 0.5 + 4.5 = 5

and 5 is significantly larger than AVSpeechUtteranceMaximumSpeechRate (1).

Is it intentional to pass a speech rate higher than the maximum?