When this setting is on, your page will automatically be optimized for Google's 'PageSpeed Insights' tool each time you publish your site. This tool analyzes your webpage and suggests ways in which you can make your page faster. A good score helps your site to achieve better ranking on Google's search result pages.
How to optimize for Google Page Speed
In the editor, click Settings and choose the option Optimize for Google Page Speed. Check the field Optimize for Google Page Speed and press the save button.