`LanguageTree:parse` is recursive, and calls
`LanguageTree:for_each_child`, which is also recursive.
That means that, starting from the third level (child of child of root),
nodes will be parsed twice.
Which then means that if the tree is N layers deep, there will be ~2^N
parses even if the branching factor is 1.
Now, why was the tree deepening with each character inserted? And why
did this only regress in #24647? These are mysteries for another time.
Fixes: #25104
6b5f44817e