Just to be sure, are you using FileMaker 12.03? (Pull down the help menu and choose about.)
one of the issues addressed in the update releases was performance issues with getting the layout to update. They didn't fix everything, but they did manage some improvements with those updates.
I just confirmed the upgrade I installed was 12.0v3.
I know well that it will not solve the problem, but just to be shure, and for your information, did you take a look at the following post ?
And few questions :• Did you try to use the classic theme on all your layouts ?• Removed unstored calculation and summary fields from your list layouts ?• Tried with a local (non-hosted) file ?• What's look like your system configuration ?
Good luck and bye, Fred