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Now multiply that by thousands of concurrent connections each doing multiple I/O operations. Servers spent ~95% of their time waiting for I/O operations.Well, no. The particular thread of execution might have been spending 95% of time waiting for I/O, but a server (the machine serving the thousands of connections) would easily run at 70%-80% of CPU utilization (because above that, tail latency starts to suffer badly). If your server had 5% CPU utilization under full load, you were not running enough parallel processes, or did not install enough RAM to do so.
Well, it's a technicality, but the post is devoted to technicalities, and such small blunders erode the trust to the rest of the post. (I'm saying this as a fan of Bun.)