my guess is, that the people who are racking so fast forward, that they have learned already several other courses of the language, and now they go on repesting it on another where you are) they go on it in the app. They go through backlog of to water … and really really fast without single mystake … they go fast forward.
I do already the same. I started maybe my fifth course of italian and fifth of portuguese. Now I chose courses with 5000 words , however I know the first 2000 words really well - of course I dash forward much faster … compared to time when I learned them first time.