ssb-backlinks is just a single index. You can query all the messages that point to a given message, and it's a single disk seek then returns results. If any message can be a root also, you must do one query, which could return N messages, then you must do N more queries. Even if those messages don't have any backlinks, and the query answers "none" that still has a cost. doing another empty query for every item in a successful query is much slower than just doing one query.
We had this design in patchwork@2 (see legacy api:
relatedMessages) it made some big threads really slow to load.