Thickbox Loading Icon In Dashboard

Hi i m running with an issue where my dashboard takes min 30 secs to load every instance. after debugging i reached to conclusion that this is thick-box Loading icon which is loaded after 20 secs. here is the image showing the loading time. https://goo.gl/OPFSKT and moreover i have already worked on heartbeat for this page still this happens. this doesn’t let me drag menus as well