I tried the app.
On my dock keyboard I am 51 WPM, with 98% accuracy.
Using the screen keyboard I dropped to 28 WPM, with 92% accuracy. Oddly though, it does not feel like I am almost twice as fast with a real keyboard. Feels just "a bit" slower on screen to me...
I don't tend to type tomes and tomes on the screen keyboard, but I find it perfectly acceptable, albeit with the total loss of tactility (half the fun of typing in my book!).