I see font text flashing / flickering when I load page first time . FOUC issue.

This is normal behavior and not a error when you load external custom fonts on any site (Google Fonts for example), you can read more here:

https://en.wikipedia.org/wiki/Flash_of_unstyled_content

This issue appears only on first page load, and then browser should cache your fonts and you will not see this issue anymore.

If you see this issue every time when you open your site pages:

• Your browser cache settings is not correct (browser does not cache fonts files)
• Your web server cache settings is not correct for font files (expiration date/time for font files does not set and browser reload fonts files every time)
• You use third party plugins that incorrectly add custom fonts to your site

You need to contact your hosting server support for help with correct hosting configuration for correct fonts files caching.

Additionall you can use third party preloader plugins that will hide this issue from users.