I wonder whether this uses the front-facing camera for blink detection, particularly at higher WPM settings. A blink typically lasts 100-400ms (per Wikipedia), and 500 WPM is .8333... words per 100ms. A blink thus seems very likely to miss a word or two once you're using this the way its creators intend. I don't see any mention of blinking or mitigating its impact on the site, though.