I've spent some effort on optimizing my sites, but most of the effort was focused on avoiding unreasonable (stupid) work. Do I need a session for every request? No, I don't! Do I need a database fetch for every access to my homepage? No, I don't! Is it a problem to actually load all of my static content in all supported languages (24) into memory and serve it from memory? No, it isn't!
I use Clojure behind nginx on the server for my sites. Oh, and I also pre-compress all static assets to Brotli, so anything that handles brotli gets a static file served directly from nginx. I also use immutable assets with unlimited caching semantics.
Really — the problem is that we've grown lax and our software has become bloated, slow, and with unreasonable code paths. If every page fetch does 12 database accesses and runs through a slow interpreter, that is surely going to be a problem.