The join field type docs contain a warning against creating multiple levels of parent/child relations:

Using multiple levels of relations to replicate a relational model is not recommended. Each level of relation adds an overhead at query time in terms of memory and computation. You should de-normalize your data if you care about performance.

However, this warning is in the last section of the page. We should move this content higher up on the page so users are aware before they start mapping their data.