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Modern CPUs are fast, and it's trivial to scale compute instances if you need that.

The database would be the bottleneck and that's got nothing to do with rails - it's all mysql.

CPU cost is the obvious driver of who will win in this space, ergo, the correct optimization is to rewrite in assembly.

Don’t they also have a ton of custom ruby stuff like their own jit?

Shopify did develop a jit, but it's part of the main ruby runtime (they did other performance improvement too, all available in the standard ruby).