I am trying to make SSH connections to multiple clients concurrently to check their availability.

I noticed that, after a specific limit, increasing thread count doesn't have an effect on total process time.

For testing purposes,
I choose a fake IP address that connection will never happen and set the connection timeout to 5 seconds.

private static void MakeConnection()
        {
            using (var client = new SshClient("1.2.3.4", 22, "test", "test"))
            {
                client.ConnectionInfo.Timeout = TimeSpan.FromSeconds(5);
                try
                {
                    client.Connect();
                }
                catch { }
            }
        }

Executed test runs for different thread & connection counts.

Here are the results:

I expect to see 5 sec for each row but somehow effective active thread counts seems like 3. Even though I spawn 8 threads, it has been processed as 3+3+2.

What might be causing this behaviour?

Simple console application to reproduce the issue:
https://gist.github.com/cankut/4efb95d2da8fdf63fb14cf762b1f953d