Optimal for whom? Any system is fairly trivial for native speakers — they just learn it. For non-native learners, the optimum level is probably the closest to

Optimal for whom? Any system is fairly trivial for native speakers — they just learn it. For non-native learners, the optimum level is probably the closest to what their native language is. Simple examples, it’s easier for a native speaker of Japanese to learn Mandarin since their is substantial overlap in vocabulary via the significant Chinese influence in the Japanese language. That said, all other things being equal, the number is probably on the low side. As a simple example, Indonesian seems to be a relatively easy language for folks coming from any language to learn, even if their native language is linguistically distant. I believe this is due to the relatively small amount of functional vocabulary.