This issue shows up for me with and without dev tools on Safari, Chrome, and Firefox
Same for me 
. I use a Sticky Header and inline links. Whenever the page loads, this flashing effects arises.
Hi @Karsten, please provide a live link to your website 
Please disable cache & optimization plugin, pause cloudflare cache (if you use cloudflare) and now check whether its work in incognito mode or not.
If still doesn’t work, please share your site url here or if you’re not comfortable to share, please send your website url directly to support email.
Thank you.