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I'm not sure yet of the full impact of RC rationalization on custom_json yet, because the real costs of operations will have changed due to recent optimizations. As examples, we have speeded up the json parser, which will make custom_json cheaper. On the other hand, we now account for signature verification costs, which will make custom_json more expensive. My guess is that, overall, custom_json will be more expensive in terms of RC than it was before. But I won't know for sure for at least a few days.