You're pointing out an important point: third-party services have to be fast, too. The call that starts the checkout does a synchronous request to Paypal Plus, which has very ugly request latencies from time to time.

The shop is indeed quite minimal at this time. The optimizations scale very well to regular ecommerce sites, though. Detail pages and categories can be cached very well, as they only change by actions of human editors. A notable exception are recommendations. If they are item-based (e.g. a-priori algorithm) they can also be cached very well. If they are user-based (e.g. collaborative filtering) that part has to be fetched from the server-side recommendation model. The same is true for search results. The heavy-hitters, i.e. the most-searched-for keywords are cached, while the others are fetched from the database. That is the part where server-side scalability and latency optimization is of utmost importance. At Baqend we support both MongoDB full text indexes and automatic separate indexing in an ElasticSearch cluster with automatic CDN and browser caching for the most frequent search results.