Based on past conversations with my mother (teacher) and a friend (linguistic researcher, can't for the life of me remember what it's actually called) I would a

Based on past conversations with my mother (teacher) and a friend (linguistic researcher, can't for the life of me remember what it's actually called) I would actually expect that to be the slowest period of learning in this area - slowest, but not that flat. So my suspicion is that it's 80% down to lack of data. (But equally possible that I'm proven wrong, as it certainly isn't my area of expertise and this knowledge comes from purely random conversations in the past.)