I'm so used to E-speak that Eloquence is incoherent to me. But, the flutter you get in E-speak in the boost mode is hindering me to achieve faster speeds. So, I'm forcing myself to get used to Eloquence.

And yes, the so called natural sounding synthesizers are not as good at high speeds. But then, SAPI5 and one core voices aren't much behind if we compare them to these so called natural voices at moderate voices. I tried a demo of Acapella, didn't like it much. It's high quality voices do sound good, and more human like, but you can't use them at high speeds. The speech becomes incoherent. There is a clatter in the background.  When it comes to high speed functionality, there is nothing better than E[speak and Eloquence.
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Regards,
Sociohack