[necovek]

Most commonly you would have a non-concurrent use of TreeMap, and then for performance reasons, someone else would come in and introduce threads in a couple of places, without ensuring that all data access (esp write) is properly guarded.

The way people structure code, it might even be non-obvious that there's a use of something like TreeMap, as it will be abstracted away into "addNode" method.

Still a red flag, since the process when introducing threads should be "ensure data structures allow for concurrent write and read, or guard them otherwise", but when the task says "performance improvement" and one's got 14x or 28x improvement already, one might skip that other "bit" due to sheer enthusiasm.