[byroot]

Weird, because if anything I keep hearing people say Ruby performance and GVL don't matter because it's all IO anyway.

Which is true to a certain extent only. When you first start optimizing a Rails app, it's true that bad queries and N+1 is where most time is wasted, but from my experience once you clear these out, the IO/CPU ratio really isn't that high, generally in the 40-60% IOs range.

[why-el]

I didn't mean to say it's all IO, that would be wrong. Rails is feature-rich, ActiveRecord for instance spends plenty of time in the CPU (manipulating its internal state). But the idea that Rails/Ruby is slow is outdated and does not jive with any monitoring and metrics we collected even in extremely busy environments. Of course one would gain some percentage of speed if say your backend is some compiled language, but that would mean possibly slower development cycles, hiring, and so on. Billion dollar companies run Rails, and, crucially, show relentless speed in execution (for instance, Stripe). If anything, Rails has the strongest case yet for performance / feature set.