Most likely, your website performance suffers from the content you have added to your website. It may happen in case of too many plugins, large not optimized images or other resources. So we suggest you to remove unnecessary plugins, install some kind of JS & CSS optimizing plugins like Better WordPress Minify and a caching plugin like WP Super Cache. Also, you can use GTmetrix tool to see what makes your website slow and optimize these parts of your website.
Can you see the image below. Create account or login pages loads before the site page content. I am using WP Rocket, did you look at the site page when choosing a product. It does the same for all pages. Please check.
It seems that you have moved all your CSS files to the footer with WP Rocket and that makes some of the elements styles be loaded after the whole page is loaded and you see this effect. Be sure that you keep all CSS files in the HEAD of the page. If you have any problems with this, contact WP Rocket support team or refer to its documentation.