I had a similar situation a few years back. Postgres has constructs that are optimization barriers. Once I understood this and reviewed the Postgres source code

I had a similar situation a few years back. Postgres has constructs that are optimization barriers. Once I understood this and reviewed the Postgres source code to determine the semantics of the relevant constructs, it was possible to get a 10X improvement for complex set of queries that had already been optimized for several years.