It’s slow

I’ve been using this theme (free version) for about a year now with 11 popular plugins (no random plugins that are junk). Anyway, it seems the more blog posts that I add, the slower the site gets. I use SiteGround hosting and they’re one of the absolute best managed hosting providers for WordPress so I know it’s not an issue with them. It’s got to be the theme. This them (with 67 blog posts) is giving me a low score of “D” every single time at Pingdom and loading at a speed of 4.6 seconds which is extremely slow in the modern web world. In addition, the page size is very large when it comes to file size as well.