Website optimization is an interesting topic. However, it is a bit too broad to effectively cover in a generic way. I can give you some resources and some generic tips, but these will only scratch the surface of what I consider to be common scenarios.
First, you mentioned that it slows down after you add visual content. I assume you mean photos, videos, etc. You can use a site like gtmetrix.com to view the size and loading time of a page, and this may help you to identify images or videos (if they autoplay especially) that can be compressed further to reduce the time it takes for visitors to download the page.
Second, you can consider optimizing your CMS. Maybe you don't use one, but in 2019 it's a fairly reasonable assumption that you're running Wordpress. If that is the case, Wordpress themselves have provided a great article on the topic: https://codex.wordpress.org/WordPress_Optimization
If it is another scenario, feel free to share any details that you're comfortable with, and either myself or the community here can offer any advice that we may have.