Well, for starters, they replace the RPC call with an in-process function call. But my point is anybody who's surprised that working with JSON at scale is expensive (because hey it's just JSON!) shouldn't be surprised.

Well everything is expensive at scale, and any deserialization/serialization step is going to be expensive if you do it enough. However yes i would be surprised. JSON parsing is pretty optimized now, i suspect most "json parsing at scale is expensive" is really the fault of other parts of the stack