It's not difficult. loadbalancer knows how many requests are in-flight to server. The OP is plainly wrong.

That's why leastconn works in the first place. And leastconn is almost always one you want. It's almost magical. GC stall on one server ? That means it isn't processing, which means every new request will go to other servers.

One server processing 2x as fast as the other ? Well, it keeps its connection count low, so it gets more of them