The Morse parameters are suggestions that happened to work well for me. You can play with parameters in this page or in the Web Morse player .
After lesson 39, there are two "post-training" or maintenance lessons that you can use to try different speeds, or just not to forget the Morse code. The option "Post-training: all chars" creates a lesson with all possible characters guaranteedly included, just like lesson 39. The option "Post-training: mostly letters" reduces the frequency of punctuation and numeric symbols to better represent the real-world frequency of characters.
The Farnsworth compression is yet another feature to aid Morse learning. The default parameters are 8 words per minute (WPM) and 20 Farns, this means that characters will "sound" like 20 WPM the inter-character spacing is increased so the effective rate is 8 WPM. If you can only "copy" in 8 WPM (because you can't type or write fast enough) but can distinguish by ear in higher speeds, the Farnsworth compression helps to "train the ear" with higher speeds.
Learning morse in a slow rate like 5 WPM and without Farnsworth compression is considered harmful because the student will have a hard time later to get used to higher speeds. She would probably have to re-train at 6 WPM, 7 WPM, and so on until reaching a useful speed in the 20-35 WPM range.