Strange performance dependency

[kelbon]

I start hyper http2 server and load it with oha (https://github.com/hatoo/oha),
if there is one connection, the number of requests per second is X,

 oha -z 2s -c 1 -p 100 --http-version=2 http://127.0.0.1:3000 

if there are more than one connections, then the number of requests per second increases by 3 times. I run in single-threaded mode. I don't understand how this is possible

 oha -z 2s -c 2 -p 100 --http-version=2 http://127.0.0.1:3000 

Nothing like this is observed with other servers - if it is run in single-threaded mode, then there is no increase in requests per second from an increase in the number of connections

Do you have explanation for this effect?

Server code:

#[tokio::main(flavor = "current_thread")] 
async fn main() {
    let addr = ([127, 0, 0, 1], 3000).into();

    let make_svc = make_service_fn(|_conn| {
        async { Ok::<_, Infallible>(service_fn(handle_request)) }
    });

    let server = Server::bind(&addr)
        .serve(make_svc)
        .with_graceful_shutdown(async {
            tokio::signal::ctrl_c().await.ok();
        });


    if let Err(e) = server.await {
        eprintln!("{}", e);
    }
}