The learning curve charts your language learning results with your language learning efforts. In a linear learning curve, a unit of time or effort yields a unit of results. In a nonlinear learning curve, at times, great efforts yield nothing or not much, (or, rarely, sometimes small efforts yield a lot)
I think languages that are highly similar to your own have a linear learning curve. You get immediate results from study and results keep improving visibly as you apply more time. Imagine a Spanish speaker learning Italian. There are unlikely to be any points in the process where there is some enormous barrier.
In Chinese and Japanese, progress moves along find until you need to memorize several thousand characters.
In all languages that aren’t especially similar to your own, the common words will be easy to learn because you can’t help but see them all the time. Then comes this enormous band of words that are used, but not every day. In my experience with Russian and Icelandic, this set of 10 to 30 thousand words is huge and a real barrier to learning how to read or understand radio news.
Inflections and grammar pose challenges at the very end of the learn process, when you begin to write for a native audience. Depending on the language, this can be another discontinuous jump in difficulty.
What does this have to do with language design?
Toki pona has a learning curve that allows for rapid progress for the first 30 hours, then progress slows to a trickle.
A conlang with a fierce phonology has a learning curve that initially is so steep, that you might never get up to the plateau where the going is easier.
Anyhow, consider what kind of learning curve you want. If the concept count is extremely low, then learning will be fast initially, then slow as you try to say more and more things with the few lexical and grammatical tools available. On the otherhand, a language that has a huge vocabulary and a huge set of grammatical devices will be a breeze to use once you master it, but the climb to the top of that peak will take years and will probably discourage most who attempt.