In fact (as I'm sure you know), one of the most beloved speech synthesizers among English-speaking blind users is a closed-source product called ETI-Eloquence that has been basically dead for nearly 20 years. (It was ported to Android several years ago, but that port was discontinued because they couldn't update it for 64-bit.) No recent speech synthesizer has quite matched its consistent intelligibility, particularly at high speeds. espeak-ng comes close, but it has a bad reputation (mostly, I think, leftover from earlier versions of espeak that really weren't very good).
Edit: Sample of ETI-Eloquence at my preferred speed: https://mwcampbell.us/audio/eloquence-sample-2021-09-25.mp3 (yes, it mispronounces "espeak")
Edit 2: To elaborate on what I mean by "mostly dead": In 2009 I was tasked with adding support for ETI-Eloquence to a Windows screen reader I developed. At that time, Nuance was still selling Eloquence to companies like the one I worked for back then. When I got the SDK, the timestamps on the files, particularly the main DLLs, were from 2002. As far as I know, an updated SDK for Windows was never released. I'm thankful for Windows's legendary emphasis on backward compatibility, particularly compared to Apple platforms and even Android.
Finally, a sample of espeak-ng (in the NVDA screen reader) at my preferred speed: https://mwcampbell.us/audio/espeak-ng-sample-2021-09-25.mp3 I use the default British pronunciation even though I'm American, because the American pronunciation is noticeably off.
Edit: Sample of ETI-Eloquence at my preferred speed: https://mwcampbell.us/audio/eloquence-sample-2021-09-25.mp3 (yes, it mispronounces "espeak")
Edit 2: To elaborate on what I mean by "mostly dead": In 2009 I was tasked with adding support for ETI-Eloquence to a Windows screen reader I developed. At that time, Nuance was still selling Eloquence to companies like the one I worked for back then. When I got the SDK, the timestamps on the files, particularly the main DLLs, were from 2002. As far as I know, an updated SDK for Windows was never released. I'm thankful for Windows's legendary emphasis on backward compatibility, particularly compared to Apple platforms and even Android.
Finally, a sample of espeak-ng (in the NVDA screen reader) at my preferred speed: https://mwcampbell.us/audio/espeak-ng-sample-2021-09-25.mp3 I use the default British pronunciation even though I'm American, because the American pronunciation is noticeably off.