Reduce reading Speed of Text-To-Speech Narration

You can use SSML tagging in the text to speech editor. Amazon has some great instructions for slowing down/speeding up the voice using the <prosody> tag here: https://docs.aws.amazon.com/polly/latest/dg/supportedtags.html#prosody-tag 

here an example to slow down the speed to 90%

<speak><prosody rate="90%">
  How do I slow down the speed? 
</prosody></speak>