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1. Running a validator is inexpensive in terms of compute, but there are 1,000,000 validators or something, which adds up to a lot of CPU usage. Of course, I think it's insanely awesome that you can run some code on Ethereum and it'll be replicated on 1,000,000 independently-operated machines, but it's not a very CPU-efficient strategy. 2. Banks doing those batch jobs probably had much higher TPS than ethereum.