I know of no way to increase the speed higher than "rate of 100."
Yours is a common complaint. I'd submit a suggestion that the top rate be increased significantly via the Feedback Hub, then post the link to same here for folks to be able to upvote it.
It amazes me how many screen reader users can understand a speech rate that leaves most of the rest of the world scratching their heads (including me)! But I also get why that's an incredibly valuable skill to develop.
Brian - Windows 10 Home, 64-Bit, Version 1803, Build 17134
The psychology of adultery has been falsified by conventional morals, which assume, in monogamous countries, that attraction to one person cannot co-exist with a serious affection for another. Everybody knows that this is untrue. . .
~ Bertrand Russell