Hello,
Your issue is not clear enough. Scroll works correctly https://gyazo.com/417b138349d7ae00c99ca7a24f9e9669
Most likely, the reason for the site speed is 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 removing 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.
Best Regards