Hi Shudock,
What you are seeing is called a Flash of Unstyled Content (FOUC).
The critical CSS configuration is empty. You can view the result in the HTML source:
<style type="text/css" id="AbtfCSS" data-abtf>
/*!
* Page Speed Optimization 2.9.10
*
* ------------------------------------
* WARNING: CRITICAL CSS IS EMPTY
* ------------------------------------
*/
</style>
The plugin includes a critical CSS editor:
https://becausechickens.com/?critical-css-editor=1
We have published a new CSS Optimization plugin that provides many new options and better optimization quality. WordPress has been blocking plugin submission for a week but the plugin can be downloaded from Github (an open source platform).
https://github.com/o10n-x/wordpress-css-optimization
If you would be interested to install the new CSS Optimization plugin, you can simply download the zip file and unzip it in the plugin directory /wp-content/plugins/.
The new optimization plugins will replace the existing Page Speed Optimization plugin. The currently missing plugins are Javascript Optimization and Progressive Web App Optimization. Those plugins will be available soon and the plugins are designed to be modules that can be used together efficiently (the optimization plugins function together as a single plugin for optimal performance).
Currently available:
– HTTP/2 Optimization
– Web Font Optimization
– CSS Optimization
Later today:
– Security Header Optimization
Next weeks:
– HTML Optimization
– Javascript Optimization
– Progressive Web App Optimization
– Page Cache Optimization
– HTTP Cache Header Optimization
The CSS editor of the CSS Optimization plugin is available with +30 themes that enables more efficient (and fun) CSS editing while it also contains many professional tools such as CSS Lint and Clean CSS (professional).

Critical CSS management is also more efficient in the new plugin so we advise to use the new plugin for CSS and Critical CSS optimization.
If you have any questions, please let us know!