Love the App But...This actually a feature suggestion.
Why don't your development team include shortcut keys to change Reading Speed? That way the user can increase or decrease the rate of reading on the fly without stopping to make adjustments using the slider interface.
In this age of media consumption optimization your extension is uniquely positioned to facilitate consumption of print media. It should however offer the same enhancements for a more personalized control of "rate of consumption" that is common with other media purveyors (eg. YouTube or Vimeo).