It looks like all the building blocks may already be present. I found this article:
https://cloud.google.com/architecture/speech-translation-android-microservice
which is about writing a speech translator using the Google Cloud components. One of the final steps is the text to speech component which, if I read this correctly:
https://cloud.google.com/text-to-speech/docs/reference/rest/v1/text/synthesize#audioconfig
has a property called “speakingRate” which defines the rate of speech output.