To make sure that the designer is running smoothly on your end, please make sure to have the following:

High Speed Internet Access. To test your internet connection go to: speedtest.net and click run test. 
Minimum Upload Speed: 2 MB (higher is better)
Minimum Download Speed: 3 MB (higher is better)
Recommended Ping: 20ms or less (lower is better)

It is also highly recommended to use Google Chrome or Firefox as your web browser.

If your internet speed is okay but you are still having problems saving your page in the designer, you may also try clearing your browser history and cache. After which, close the tab and try relaunching the designer's website in a new tab.