Attempt to modify site module and publish website takes too long time and produces the error:
A script on this page may be busy, or it may have stopped responding. You can stop the script now, or you can continue to see if the script will complete.
Software issue #PPB-4331 which has been fixed in Parallels Plesk 12.0.18 MU#29.
Install Plesk MU#29 on the server.