Someone replied but deleted right when I was posting this answer, so I'm replying to myself:

What I didn't understand was why the listed performance optimizations couldn't be implemented in the monolith itself and ensued the development of a new application, which is still Ruby.

In a production env, the request reaches the Rails controller pretty fast.

I know for a fact that the view layer (.html.erb) can be a little slow if you compare it to, say, just a `render json:`, but if you're still going to be sending fully-rendered HTML pages over the wire, the listed optimizations (caching, query optimization and memory allocation) could all be implemented in Rails itself to a huge extent, and that's what I'd love to know more about.