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
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.
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