I'm comparing <sub> tags in SSML with iOS16 beta.
AVSpeechSynthesizer adds short pause before and after tag, where Google TTS does not. https://cloud.google.com/text-to-speech/docs/ssml Also it pronunces "." as "period".
let uttreance = AVSpeechUtterance(ssmlRepresentation: "<speak> I can also substitute phrases, like the <sub alias=\"World Wide Web Consortium\">W3C</sub>. </speak>")
The same happens with <phoneme> tags. I think it shouldn't have extra pauses.