How fast can an ActiveRecord SQL query run?

Your database monitoring (perhaps Amazon RDS performance insights, Skylight, or the Postgres pg_stat_activity view) is telling you that one particular query in your Rails app is causing heavy load. Unfortunately, it already seems pretty fast. What are the odds that you can get it to run any faster? I had a case where the query generating the heaviest load in an application took 20 milliseconds. It was also the fastest query in the application; some queries in the same application take entire seconds to run! Surely the fastest query in the system had little room for optimisation?.