To give C1 a chance and let things go fluently: there's a little wheel right beside the RGB values in the top border (under the toolbar, if you use it) indicatiing that C1 is "working". Wait until it disapears. Too, as to the rendering of thumnails and focus mask, hiding the viewer helps a lot. With the browser expanded you'll see if C1 is ready with the rendering of previews or not. First start editing when C1 is ready. That's of course only a workaround until V5 is as rock solid as V3... if that will ever happen.