Third, Keybr.com lets you introduce as few keys as possible to the lesson, adding more keys automatically when it decides that you are proficient at the current level. When you only start learning it generates lessons with words from a very small alphabet of the most frequent letters. When your typing speed for every key in that alphabet reaches certain threshold, the algorithm adds the next most frequent letter to the alphabet. And so on, until the next letter. This way you will learn the most frequent letters first, and the least frequent ones later.
It is more important to have a low error percentage than a higher speed. The low error percentage will be your best guarantee for success in the following lessons. You can follow your progress at the statistics page (just press the button 'Statistics' at the upper right corner) You should strive to do all lessons flawless (or at least with max two percent error) before you go on to the next lesson. If you learn to type flawless a high typing speed will automatically ensue. By the way, the error statistics will only save your best results.