The author of the article encountered an issue with slow PostgreSQL queries in their database, which affected only a small percentage of users. The problem was caused by a primary key lookup query that became slow due to a large number of keys being used. To fix this, they made a simple one-line change in the query from `ANY(ARRAY[...])` to `ANY(VALUES(...))`, which allowed Postgres to use the index more efficiently and resulted in a significant speedup from 20 seconds to 0.2 seconds. The change was validated using tools like Datadog, which provided visibility into the performance of the query before and after the change was made.