@davidculley my experience with Ruby on Rails application mostly involves running both Mastodon and Gitlab on my servers.
In both the cases, what I’ve noticed is that it’s not the language itself that is slow and heavy (Ruby’s weight is comparable to that of e.g. Python), but Sidekiq.
Sidekiq is the standard framework used by Ruby on Rails application to schedule and run asynchronous jobs (processes, threads…), kind of akin to what php-fpm does for PHP.
In my experience, it’s hard to configure properly, and when not configured properly it ends up with endless pools of active jobs doing all kind of things and sucking up all resources you give to them.