Here’s my quick guide to fixing slow queries:

1. grab your query.
2. run EXPLAIN ANALYZE <query>
3. If there’s a Parallel Seq Scan, check the Filter value (which column are we checking) - you should likely try adding an index.
4. run CREATE INDEX ON <table>(<column>);
5. run EXPLAIN ANALYZE <query> again to check for improvements

Note that EXPLAIN <query> will only do a rough analysis, while EXPLAIN ANALYZE <query> will run your actual query, and analyse it.

If you want to ANALYSE EXPLAIN an INSERT query, do this:

BEGIN;
EXPLAIN ANALYZE INSERT ...;
ROLLBACK;

A simple ROLLBACK will help you here.

Rewriting the query will typically not do much - it might, but the chance it relatively small vs slapping an index on it.