After auditing dozens of Rails apps, here are the 5 problems I see in almost every single one:

1. N+1 queries hiding in plain sight. You added includes in the obvious places but your serializers, mailers, and background jobs are still firing hundreds of extra queries per request.

2. No database indexes on polymorphic associations. Rails doesn't add these by default. One composite index can turn a 2-second page load into 50ms.

3. Fragment caching without Russian doll nesting. You're caching views but busting the entire cache on every update instead of letting inner fragments survive.

4. Running Sidekiq jobs synchronously by accident. perform_now calls buried in service objects, tests leaking into production configs, or retry logic that blocks the web process.

5. Oversized payloads from to_json on ActiveRecord. You're serializing entire objects with associations when the frontend only needs 3 fields.

Every one of these is a quick win. Fix all five and you'll cut response times by 60-80% without touching infrastructure.

If you want us to find these (and the ones you can't see) — that's exactly what our Rails Performance Audit does.