It does take a long time, and also "real life" immersion, to learn a language deeply. It's the immersion part that Duolingo doesn't do well if at all. So it has

It does take a long time, and also "real life" immersion, to learn a language deeply. It's the immersion part that Duolingo doesn't do well if at all. So it has to artificially slow down those things it does do in order to keep subscribers.