White flash while loading pages

Hello,

We’re using AO with criticalcss add-on, along with WP Fastest Cache and keyCDN.
The problem is that we’re seeing a “white flash” when loading any page on our website. It was less than half of the second but happens on different devices/IPs.

We have read through this topic and even turn off Google Optimize Code on our Google Tag Manager, however, it didn’t help.
https://www.ads-software.com/support/topic/screen-flashes-for-5-sec-on-firefox-less-noticeably-on-chrom/

Would you mind checking and let us know the solution?
Regards,
Mark

P.S: Here is our app setting
Aggregate JS-files? (x)
Also aggregate inline JS? (x)
Aggregate CSS-files?(x)
Inline and Defer CSS? (x)