The following steps describe how to change the speech rate of the read function in your device's Settings.

Please refer to the official Google tutorial if you are running Android 7.0 and up here:
https://support.google.com/accessibility/android/answer/6006983?hl=en

The following instructions are for devices running Android 5.0 and up.

1. Go to your devices Settings and tap Language & input

2. Tap Text-to-speech output


3. Tap Speech rate


4. Select desired speed