The problem is mostly caused by an enormous number of products and images and products in your mega menu. Your average page size is ~20MB and your website is not usable at all. You need to have not more than 2-3MB so need to reduce the number of images by ten times. You can temporarily remove all HTML blocks from your header test how it works. https://gyazo.com/6b6a285c004ee3d002001080ca8b471e

Also, keep all plugins turned off while testing. Let us know about the result so we can check it ourselves.