10 or 20us can be longer than some IO operations. But the direct issue with OS threads is simply that you can't realistically have too many of them, and so they

10 or 20us can be longer than some IO operations. But the direct issue with OS threads is simply that you can't realistically have too many of them, and so they limit your level of concurrency. If the number of threads you can maintain is more than the average request rate times the average latency of each transaction (if it were processed on a single thread), then you're all good; if it's less, then your server will crash as requests pile up. Fear doesn't play a role here, just Little's law.