Just boils down to not optimising until you need to. Start with a 3 tier web app (unless your requirements lead you to another solution), then start with read replicas, load balancing, sharding, redis/RabbitMQ etc
In almost all performance areas -- gaming, PCs, autos, etc -- there are usually whole publications dedicated to performing benchmarks and publishing those results.
Are there any publications or sites which implement a few basic applications against various new-this-season "full stacks" or whatnot, and document performance numbers and limit-thresholds on different hardware?
Likewise, there must be stress-test frameworks out there. Are there stress-test and scalability-test third-party services?
Fossil SCM is a great example of a sqlite application that has stood the test of time. I don't know what sqlite.org's traffic is like, but it's not tiny and it runs on a tiny VPS without issue(and has for years now).
True, but it's great that they have a certain comparable hello-world set, so you can just take the 2-n benchmarks for the tech you're interested in and either ungamify it or just implement your own prototype and measure. The benefit of already having a non-random-tutorial-from-google app is a huge win to me personally.