We handle a lot less traffic than Shopify (~15-20 million reqs a day) but ActiveRecord isn't a bottleneck and I wouldn't expect to to be [...unless you are talking about something other than the performance of AR as a body of code?]
Instead of "Rails doesn't scale" we should say "Rails runs on Ruby which means that it will consume significantly more CPU and more memory* compared to something else"
In my case, 1 extra server (my estimate) was a small price to
pay for developer happiness.
I love nearly everything about Pine, but I wish that Dovecot's IMAP searching was faster.