UI freezes when sending image over TCP

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C#

WinForms

networking

TCP/IP

, +

I write this code to send screenshot to multiple connected clients.Works fine on clients end, but freezes the UI of application on server side.I don't understand what cause that problem.

public void LoopClients()

       {
            while (_isRunning)
            {

                TcpClient newClient = Server.AcceptTcpClient();

                Thread t = new Thread(new ParameterizedThreadStart(HandleClient));
                t.Start(newClient);
        }
        }
        public void HandleClient(object obj)
        {
            TcpClient client = (TcpClient)obj;

            BinaryFormatter binaryformatter = new BinaryFormatter();
            while (client.Connected)
            {

                MainStream = client.GetStream();
                binaryformatter.Serialize(MainStream, GrabDesktop());

            }
        }
        private static Image GrabDesktop()
        {
            System.Drawing.Rectangle bound = Screen.PrimaryScreen.Bounds;
            Bitmap screenshot = new Bitmap(bound.Width, bound.Height, PixelFormat.Format32bppArgb);
            Graphics graphics = Graphics.FromImage(screenshot);
            graphics.CopyFromScreen(bound.X, bound.Y, 0, 0, bound.Size, CopyPixelOperation.SourceCopy);
            return screenshot;
        }

I call the loopClient() function in timer and than timer in button to run the program.
Any suggestion to improve the code or fix t resolve the problem would be great help.

What I have tried:

I have searched many solution but noting is quit helping in any ways.