Hello João Barros,

The developers are aware of this phenomenon.  In the meantime you can reduce the risk of program hangs by adhering to the following:

• Try to design so that the number of empty cells on the page is kept to an absolute minimum
• Avoid using empty Objects...  instead fill each Object with some content

Some users also switch off the responsive features within WebSite X5 until they have finished editing and are ready to upload to the server, whereupon they switch responsive functionality back on again.  If you want to try this then go to Step 1 General Settings > Responsive Design (tab) and toggle the checkbox 'Enable responsive website' on or off as required (see the screenshot attached).

Hope this helps...

Paul