Latency doesn't affect throughput. As soon as you fire off a call to an external vendor, your server can work on processing the next request. It doesn't matter

Latency doesn't affect throughput. As soon as you fire off a call to an external vendor, your server can work on processing the next request. It doesn't matter if that call takes 10 ms or 5 seconds. Even if it took 30 seconds, that means you need to be able to handle 12k concurrent requests (almost all of which are asleep), which is nothing.