When you pass lesson 15 your load increases, because you have more and more characters to select from. Above that new characters are less exercised in a fixed lesson length.
So above that lesson exercise also fixed custom codegroups, containing only the 5 or 6 last learned characters.
Many thanks, very good tip. I grounded at lesson 38, due to ,/?.= b6 h5 v4 decoding problems with the required speed.
Followed your advice, and right now I broke the barrier and arrived at the starting of lesson 40 speed 20/10