The page is loading for me quite fast, hardly notice any kind of flicker.
You can try to do some optimization on the site, first try to move the custom css from the customizer to your child theme css file in /wp-content/themes/puresimple-child/style.css
See if that helps.
You can also try to force the logo height to be fixed so the header have a preset height and it won’t be “moving the rest of the page down” until your logo image loads fully loads, try this CSS
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