I'm not, otherwise something like 0.1% would be a reasonable upper bound. Even if you figure each order involves 20 async steps, that would be ~8k steps per sec

I'm not, otherwise something like 0.1% would be a reasonable upper bound. Even if you figure each order involves 20 async steps, that would be ~8k steps per second, which is nothing for a modern server. My old i5 can do 70k simple requests/second (the first type you describe) on the JVM with the app and database on the same computer (so contending for CPU time). A modern server CPU is ~15x more capable than mine without getting into multi-socket. Put another way, a dual socket Xeon platinum can give you 120 cores. To do 400 orders/second on such a machine, you need to do less than 300 ms of core-time per order, which is an insanely large number. The things you mentioned spend almost all of their time waiting.