I have read several discussion on the matter and managed to understand a bit, but still having some issues solving the performance problem when converting from Thread class to ThreadPool class

Details:

I have build a tcp server, with educational purpose(the task should not be with async methods), which is accepting client connections and create a new thread for each client. With this method the application is executed for less than a second, but when decided to move to a next level solution like thread pool my performance dropped to 40-50 seconds for just a hundred clients and i just send a 2048 byte buffer, receive it and close.

The first 12 threads are very fast, most probably because my cpu is 6 cores 12 threaded and after that the threads start to experience a delay. I am open to solution ideas and structure approach.

Server code:

public void OnSocketReceive()
    {
        while (!this.exitServer)
        {
            if (!tcpListener.Pending())
            {
                Thread.Sleep(10);
                continue;
            }

            TcpClient client = tcpListener.AcceptTcpClient();

            IManageConnectedUser chatLogic = new ManageConnectedUser(connections, welcomeMessage);

            //Thread clientThread = new Thread(new ParameterizedThreadStart(chatLogic.OnClientConnection));
            //clientThread.Start(client);

            ThreadPool.QueueUserWorkItem(chatLogic.OnClientConnection, client);
        }

More clarification

By far with the debugging I have done and the discussions I have read I made the conclusion that the problem is the blocking code in OnClientConnection function and more specifically in the inner function which is stringCreateHandler. Where i receive this error: System.Threading.ThreadInterruptedException: Thread was interrupted from a waiting state on line 39, which is the Thread.Sleep(10);

public string stringCreateHandler(TcpClient client)
    {
        StringBuilder sb = new StringBuilder();
        try
        {
            do
            {
                if (client.Available > 0)
                {
                    while (client.Available > 0)
                    {
                        char ch = (char)client.GetStream().ReadByte();

                        if (ch == '\r')
                        {
                            continue;
                        }
                        if (ch == '\n')
                        {
                            return sb.ToString();
                        }

                        sb.Append(ch);
                    }
                }

                Thread.Sleep(10);
              
            } while (true);
        }
        catch (Exception e)
        {
            Console.WriteLine(e);
            throw;
        }

    }

I have checked the rest of the code i don't think there is more blocking code, but i am giving a link to the project https://github.com/nikolaymih/Chat-Project/tree/master/ChatProjectNewThreads The only difference there is the Thread class instead of ThreadPool but this is just for orientation