However, in basket, I clicked "Zur Kasse" to checkout, and the response took 11.08 seconds to get to the checkout page per Chrome network tab. Repeated multiple times with same result.
Also, I don't see any product detail pages, categories, search result pages, or anything other than 7 products on the homepage that add directly to cart. Does this strategy scale to regular ecommerce site levels?
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.
The rest of it is so well done, but I have to wonder how many sales were lost because of this checkout issue.
During our testing, Paypal had latencies of ~100-500ms but these new latency outiers of 10 seconds are a no-go. We will definitely change the code to make that API call asnychronous. Generally Paypal Plus is still quite buggy and not well documented.