This also happens on your demo page so it must be something to do with the slider.

When you first enter the site. a super large image of the 1st product is seen before it scales down. It basically fills up the entire page for 1/4second.

Please find a way to fix that. It makes a site seem very unprofessional.

thanks

https://www.ads-software.com/plugins/tc-woocommerce-product-slider/