I am experimenting with SQL query performance for SELECT queries on large tables and I observed that changing/increasing the degree of parallel hint doesn't give the expected performance improvement.
Why do you believe you are changing the degree of parallelism? PostgreSQL does not have parallel hints (or any hint in comments), so you are just changing a comment in the queries, which changes nothing at all in the execution plan.
Unless you are not using vanilla PostgreSQL or you have some extension in place, in which case you didn't provide enough information.
