The assemption that the ability to optimize something is a trivial nicety vs an essential tool to keeup your business running and growing boggles my mind every time I'm engaging with the Rails community.
I get it that most startups in the early phase don't care if a request is taking 200ms or 1s and should just go with whatever is easiest to implement. The tendency to generalize this thought to mean that nobody ever needs to care about these problems is crazy though.
Rails can scale horizontally just fine for a huge, huge majority of business cases. If you have a business critical use case that truly needs however many thousands of things done in a tight loop inside a single process...obviously Rails is not the tool you should be using.
I did not say "the ability to optimize something is a trivial niceity". I said "concurrency often gets in the way more than it helps unless you're trying to optimize something". I'm not sure if you're purposefully misrepresenting my words, but it does feel like you are especially given the "so you never make HTTP requests" comment you started this thread with.