MQTTNet slow performance with 10K clients

Question

I ran few performance tests with MQTTNet and got unexpected results Looking for help to see if there are some configuration changes that I missed or there are MQTTNet implementation limitation. Environment: I have 3 processes on one box : Sender -> Server -> Subscriber

Tests:

 Messages              Sender                                 Server      Subscriber       Time   
  100K                  1 instance sends 100K messages     ->  1       ->   1 instance     30sec  
  100K (1000*100)       1k instances sends 100 messages    ->  1       ->   1 instance     32sec 
  100k (10000*10)       10K instances sends 10 messages    ->  1       ->   1 instance     340sec  

I see that moving from 1 instance to 1000 do not significantly impact performance, but moving to 10K senders (1 process with 10k client instances) significantly impact performance.

In last test all messages delivered to server in 50sec and after server sends messages to subscriber. after 50 sec CPU dropped to 25%

Server start

var optionsBuilder = new MqttServerOptionsBuilder()
                .WithConnectionBacklog(10000)
                .WithDefaultEndpointPort(8883)
                .WithDefaultEndpointBoundIPAddress(IPAddress.Parse("xxx"))
                .WithApplicationMessageInterceptor(context =>
                {                    
                    Interlocked.Increment(ref cnt);
                }

                )
                .WithMaxPendingMessagesPerClient(100);                   
            mqttServer = new MqttFactory().CreateMqttServer();
            await mqttServer.StartAsync(optionsBuilder.Build());

Client Start

var options = new MqttClientOptionsBuilder()
                .WithClientId(name)
                .WithTcpServer("xxxx", 8883)
                .Build();
            var factory = new MqttFactory();
            mqttClient = factory.CreateMqttClient();
            mqttClient.UseApplicationMessageReceivedHandler(e =>
            {
               Interlocked.Increment(ref cnt);               
            });
            // for receiver 
             mqttClient.UseConnectedHandler(async e =>
                {
                    Console.WriteLine("### CONNECTED WITH SERVER ###");
                    await mqttClient.SubscribeAsync(new TopicFilterBuilder().WithTopic("my/topic").Build());
                    Console.WriteLine("### SUBSCRIBED ###");
                });

MQTTNet version 3.0.11,

OS Windows 10

Why with 10K clients I observe so significant degradation while almost no degradation with 1000 clients. Can I miss some configuration ?