Rehub and childthemes don’t all get good scores in google pagespeed. Their recommendations are a good indicator for optimizing for best user experience, and for SEO.
Looking for alternatives I found the theme by affiliatetheme.io which doesn’t seem to be based on woocommerce, and lacks some features like wishlists and dynamic product compare. But it looks very nice and clean, and has good scores in google pagespeed, pingdom and gtmetrix.
That doesn’t mean it will perform well on a website with 800,000+ products. I don’t have access to such a site, so I’m wondering if you would consider testing that theme.