Heartbeat not closing session

Hi all,
I'm building an application that has to do some stuff in background. To accomplish this I'm using java Timers and TimerTask. 
Since these tasks can run for a while, I want to stop them if the user close the browser but I have a problem with Heartbeat interval.
I've set it to 2 and I've read Vaadin waits 3 heatbeats to fail before starting any UI clean up.
Well, my application looks like this

public class WebUI extends UI implements DetachListener
{
    @WebServlet(value = "/*", name="WebUIServlet", asyncSupported = true)
    @VaadinServletConfiguration(productionMode = false, ui = WebUI.class, heartbeatInterval = 2)
    public static class WebUIServlet extends VaadinServlet implements SessionDestroyListener
    {
        @Override
        protected void servletInitialized() throws ServletException 
        {
            super.servletInitialized();
            getService().addSessionDestroyListener(this);
        }
        
        @Override
        public void sessionDestroy(SessionDestroyEvent event) 
        {
            // cleanup task(s)
        }
    }
        
    @Override
    protected void init(VaadinRequest request) 
    {
        addDetachListener(this);
        // other code...
    }
    
    @Override
    public void detach(DetachEvent event) 
    {
        // cleanup task(s)
    } 
}

My questions:
1) Where should I put my cleanup task(s) code? Into sessionDestroy() or detach() method ?
2) Why if I've set heartbeatInterval to 2, instead of wating 6 minutes for UI cleanup, those methods get called after 25/30 minutes?