> The only Rails projects that I worked on that never had performance problems are the ones that never reached any scale. All Rails projects that gained traction that I worked on, needed serious refactorings, partial rewrites, tuning and tweaking to keep 'em running.
You'll be hard pressed to find any stack that doesn't require this.
A big problem with rails, though, is how easy it makes it to "do the bad thing" (and in rare cases, how hard it makes it to do the "good" thing).
A has_many/belongs_to that crosses bounded domains (adds tight coupling) is a mere oneliner: only discipline and experience prevents that. A quick call to the database from within a view, something that not even linters catch, it takes vigilance from reviewers to catch that. Reliance on some "external" (i.e. set by a module, concern, lib, hook or other method) instace-var in a controller can be caught by a tighly set linter, but too, is tough.
Code that introduces poor joins, filters or sorts on unindexed columns, N+1 queries and more, are often simple, clean-looking setups.
`Organization.top(10).map(&:spending).sum` looks lean and neat, but hides all sorts of gnarly details in ~three~ four different layers of abstraction: Ruby-language because "spending" might be an attribute or a method, you won't know, Rails, because it overloads stuff like "sort", "sum" and whatnot to sometimes operate on data (and then first actually load ALL that data) and sometimes on the query/in-database. It might even be a database-column, but you won't know without looking at the database-model. And finally the app for how a scope like top(10) is really implemented. For all we know, it might even make 10 HTTP calls.
Rails (and ruby) lack quite some common tools and safety nets that other frameworks do have. And yes, that's a trade-off, because many of these safety nets (like strong and static typing) come at a cost to certain use-cases, people or situations.
Edit: I realize there are four layers of abstraction if the all-important and dictating database is taken into account, which in Rails it always is.
You'll be hard pressed to find any stack that doesn't require this.