for (int i = 1; i<=Q; ++i) {
  Task.Factory.StartNew(() => {
    doWork(n);
  });
}

for (int i = 1; i<=Q; ++i) {
  new Thread(() => {
    doWork(n);
  }).Start();
}

public int GetNum() {
  return 42;
}

[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, ConcurrencyMode = ConcurrencyMode.Multiple, UseSynchronizationContext=true)]

D>for (int i = 1; i<=Q; ++i) {
D>  Task.Factory.StartNew(() => {
D>    doWork(n);
D>  });
D>}
D>

class MyServiceImpl
{
    public Task<int> GetAnswerToTheUltimateQuestionOfLife()
    {
        return TAsk.Factory.StartNew() => {Thread.Sleep(10000); return 42;}
    }
}

D>public int GetNum() {
D>  return 42;
D>}
D>

D>[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, ConcurrencyMode = ConcurrencyMode.Multiple, UseSynchronizationContext=true)]
D>

[ServiceContract]
public interface IThread {
    [OperationContract]
    void Start(int nTasks, int nActions);

    [OperationContract]
    byte[] Status();
        
    [OperationContract]
    int StatusSimple();
}

[ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, ConcurrencyMode = ConcurrencyMode.Single, UseSynchronizationContext=false)]
public class WebThread : IThread {
    public WebThread() {
        //Thread.CurrentThread.Priority = ThreadPriority.Highest;
        //Process.GetCurrentProcess().PriorityClass = ProcessPriorityClass.RealTime;
        graphics = Graphics.FromImage(bitmap);
    }

    public void Start(int nTasks, int nActions) {
        bitmap = new Bitmap(Width, (nTasks+1)*10);
        graphics= Graphics.FromImage(bitmap);
        x=0;

        for (int i = 1; i<= nTasks; ++i) {
            var n = i;
            Task.Factory.StartNew(() => {
                doWork(n, nActions);
            });
            //new Thread(() => {
            //  doWork(n,qActions);
            //}).Start();
        }
    }

    public byte[] Status() {
        using (var ms = new MemoryStream()) {
            lock(bitmap)
                bitmap.Save(ms, ImageFormat.Png);
            return ms.ToArray();
        }
    }
    public int StatusSimple() {
        return n++;
    }
    private int n=0;//номер обращения

    private void doWork(int n, int qActions) {
        Thread.CurrentThread.Priority = ThreadPriority.Lowest;//наполовину помогло
        for (int j =0; j<qActions; j++) {
            for (var k=0; k<1000000; ++k) 
                v1=1/(j+rnd.NextDouble());
            log(n);
        }
        logEnd(n);
    }

    private void log(int n) {
        lock (bitmap) {
            graphics.DrawLine(new Pen(Color.Black), x, n*10, x, (n+1)*10-1);
            x++;
        }
    }
    private void logEnd(int n){
        lock (bitmap) {
            graphics.FillRectangle(new SolidBrush(Color.LightBlue), x, n*10, Width-x, 10);
            x++;
        }
    }

    private int Width = 1700;
    private int x = 0;
    private Bitmap bitmap = new Bitmap(1, 1);
    private Graphics graphics;
    private readonly Random rnd = new Random();
    private double v1;
}

Single

The service instance is single-threaded and does not accept reentrant calls. If the InstanceContextMode property is Single, and additional messages arrive while the instance services a call, these messages must wait until the service is available or until the messages time out.

D>[ServiceContract]
D>public interface IThread {
D>    [OperationContract]
D>    void Start(int nTasks, int nActions);

D>    [OperationContract]
D>    byte[] Status();
        
D>    [OperationContract]
D>    int StatusSimple();
D>}
D>