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?