问题：在 iOS 7.x 和 iOS 8.x 中，此代码运行良好，但在 iOS 9.x 中，与 iOS 7 和 8 中的语速相比，语速越来越慢。
self.synthesizer = [[AVSpeechSynthesizer alloc] init];
self.synthesizer.delegate = self;
AVSpeechUtterance *utterance = [[AVSpeechUtterance alloc] initWithString:[NSString stringWithFormat:@"Hey %@, please choose places to explore or select excursions to see our custom crafted deals",[defaults objectForKey:@"USERNAME"]]];
utterance.voice = [AVSpeechSynthesisVoice voiceWithLanguage:@"en-US"];
utterance.rate = 0.10;
[self.synthesizer speakUtterance:utterance];