The indexed FKs have nothing to do with performance; increased performance MIGHT be a side-effect, but - as you point out - it might as well not be such an effect. The idea of indexing FK columns has to do with the locking mechanisms.

In order to be sure that you can delete a parent record you must also make sure that there are no children belonging to that parent. This can only be achieved by actually reading the child table(s). When you don't have any usable index on the child table, then you must read all records - and they get locked at the same time, just because they are part of an updating (read-write) transaction.

This is very common truth för many relational databases, including SQL Server, Oracle, Firebird and others, and it has so been since "ancient" times.

P.S. I thought it was common knowledge... 😉