SD Voice for Posts turns a blog post into an MP3, and it does the work on your own machine, not on somebody's server. So the speed is your machine's speed. This page measures it. Nothing is installed, nothing is sent anywhere.
The estimate above is read from your hardware. For a real number the page can download the same voice this plugin uses and time one sentence on your machine.
Nothing here is uploaded. The voice model is downloaded from Hugging Face into your browser, and the measurement runs locally. There is no account, no key, and no server of ours involved at any point.