Before I answer your question, I want to make you aware of one of the handiest commands for a new NVDA user when it comes to finding the keyboard shortcuts used to control the vast majority of "commonly used" features:   NVDA+N,H,Q  (which is INS+N,H,Q in desktop keyboard layout) will cause the Commands Quick Reference to open in your default web browser.  The question you're asking is about one of the common features/tweaks, and the keyboard shortcuts for changing the reading rate, is found there under Synth Settings Ring, copied here:

Synth settings ring

Name	Desktop key	Laptop key	Description
Move to next synth setting	NVDA+control+rightArrow	NVDA+shift+control+rightArrow	Moves to the next available speech setting after the current, wrapping around to the first setting again after the last
Move to previous synth setting	NVDA+control+leftArrow	NVDA+shift+control+leftArrow	Moves to the next available speech setting before the current, wrapping around to the last setting after the first
Increment current synth setting	NVDA+control+upArrow	NVDA+shift+control+upArrow	increases the current speech setting you are on. E.g. increases the rate, chooses the next voice, increases the volume
Decrement current synth setting	NVDA+control+downArrow	NVDA+shift+control+downArrow	decreases the current speech setting you are on. E.g. decreases the rate, chooses the previous voice, decreases the volume

You'd have to get yourself to the speech rate for your chosen synth using one or the other of the first two commands, then increment/increase or decrement/decrease the rate using the third and fourth commands, respectively.
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Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042  

The depths of denial one can be pushed to by outside forces of disapproval can make you not even recognize yourself to yourself.

       ~ Brian Vogel