Flash of unstyled content

Hello great CatchThemes i try to use a plugin called (Above the fold optimization) https://it.wordpress.org/plugins/above-the-fold-optimization/ I installed on my testing website tecnologino.com After i enabled Optimize CSS Delivery my tecnologino.com with catchbox pro 4.7 appear "Flash of unstyled content" you can see live on refresh the page. How can i optimize the CSS on my catchbox pro for better PageSpeed Insights ? without this "flash second" problem? The plugin work, the website work too and css was moved above the fold but i have this problem on refresh the page for just 1 milliseconds. Test is here: http://www.tecnologino.com/?compare-abtf=d8d585da1d99cfbaf8c6e16cb6dd6576