If you are working on a very large form, the Designer may need more time to update the screen than usual.
Try this to shorten the wait:
- Close the Preview screen for a while (select View – Hide Preview or press X in the upper right corner).
When you need it again, select View – Show Preview.
- On the main form element, right-click to get the menu and select Collapse All.
Then open manually only the elements (pages, grids, etc.) that you are going to be working in.