Being presented a sequence of random, isolated nouns, verbs, and adjective doesn't seem like typing to me at all, or at least no kind of typing I've done in practice.
Example sequence: swat repetitious defensive tousled pimpled rerun closure poke weigh
This seems more useful for improving, say, my typing dexterity and my vocabulary. :P
Here's a fun one for programmers: http://typing.io/
Edit: I see now I can change the number of words displayed at the bottom of the page. Not great affordance.
That's the point. It's to build up muscle memory in the fingers so you use the correct fingers for typing. It's to bypass bad habits you may have learned with regular typing.
It is much harder to type those typing drills, but they do help when you're building your rhythm and speed.
That would help me, the user, know what to expect and therefore avoid an unpleasant first-time experience. It would also help differentiate this typing tutor from all the other ones.
I'm a reflective guy so I was able to think of a scenario where this tool was useful, but still assumed it was meant for another scenario. Most users will just pattern match this against all the other typing tutors they've used, find it unusual, and never come back.
Now "3" just seems like an arbitrary decision. Why 3? Why not 2 or 4 or 5?
Instead, I might take the fact that everyone here on HN mentioned it* as a signal that they're (1) anxious to change it and (2) can't find a way to relieve that anxiety.
(2) is happening because the attention is (rightly) focused on the typing bit of the screen.
What if there were a slider from 1 to, say, 5 right below the the typing area? It'd be easy to ignore but users would notice it as soon as they felt anxious about changing the number of words.
If you made it tie in dynamically with the value at the bottom of the page, it might also be an opportunity to "teach" your user to look at the bottom of the page for more advanced configuration options. Better yet: have a smaller version of the slider down there and connect the two. Then there would be literal movement to draw the users' attention to the bottom section of the page.
Other design nit: the top bar representing time and the bottom bar representing position shouldn't both be skinny orange lines. I realize the bottom bar is slightly thicker, but it still implies a connection where there isn't one.
A small non-orange triangle pointing up would be a good icon for the bottom bar, indicating "you are HERE." When a user makes an error, the triangle could advance and in addition to the letter turning red, it cloud leave a tiny red "X" on the same line as the triangle beneath the mistyped letter.
*: With the huge caveat that people on HN are lousy whiners and typically can't see the forest for the trees. Depending on how you feel about my feedback, I'm happy to be included in that bunch this time around. :)
Aaaaand this is why I use an IDE, so I don't have to type "protected" 100 times.