What you're describing sounds like vanilla async concurrency. I seriously doubt 'most applications' use the one-thread-per-request model at this point in time,

What you're describing sounds like vanilla async concurrency. I seriously doubt 'most applications' use the one-thread-per-request model at this point in time, most major frameworks are async now. And it's not a silver bullet either, plenty of articles on how single-thread is sometimes a better fit for extremely high-performance apps. After reading all of you responses, I still don't see how you think your learnings apply to Discord. They would not be able to fit the indexes in memory on MongoDB. They can't batch reads or writes at the application server level (the latency cost for messaging is not acceptable). Millions of queries happen every second, not one-off analytical workloads. It seems these two systems are far enough apart that really there is no meaningful comparison to be made here.