Most slow queries are a missing index away from fast. A deep-dive on indexing Postgres properly — what to index, composite column order, partial and covering indexes, reading EXPLAIN, and finding the indexes you don't have (and removing the ones you don't use).
A catalog page that's fast for ten users can fall over for ten thousand. A deep-dive on the layers of caching that keep it fast — fragment caching, HTTP/CDN caching, low-level caching — and the hard part: invalidation and stampedes.
A product catalog is mostly images, and naive image handling will sink it. A deep-dive on building an image pipeline that scales — keeping originals, generating variants in the background, offloading to a CDN, and not melting your web servers.
The N+1 query is the most common performance bug in Rails apps, and it hides perfectly in development. A deep-dive on what causes it, the difference between preload/eager_load/includes, counter caches, and how to catch it automatically.