Reports of Ruby slowness are often exaggerated. Around the time GH was built that was certainly true, but in the ways that matter Ruby is more than fast enough nowadays, even at scale. (Shopify is a rails monolith, as is intercom and a bunch of other massive services) Compute is/was pretty cheap and the economics of ROR mean that it's ultimately not that expensive to run. I think GH's slowness is a mixture of mismanagement and issues on the frontend.

But for sure a systems language is going to be far faster on paper and Rails is far from perfect and does have some performance foot guns you need to avoid. And yeah, architecture is everything.