> for the average CRUD-heavy SaaS app, the "boilerplate tax" of writing eighty lines of repository code for a simple related insert might eventually cost more in development velocity than the performance gains are worth.

Perhaps, but IME this kind of thing is much more often the cause of poor performance in CRUD apps than the frontend frameworks that are usually blamed. I have been able to make very snappy SaaS apps by minimizing the number of queries that my API endpoints need to perform.

I've also found that the ORM mainly reduces boilerplate for Insert/Update operations, but often adds a very significant amount of boilerplate for read queries. We ended up using a very lightweight orms for simple inserts / upserts and writing raw SQL for anything else.