Any related post query made to the server is going to impact performance and there isn’t anything the theme does in regards to how long your server takes to complete the request. The only solution i am aware of is Biblio related posts which does all the work on their own server and loads the content after page load – but there is a limit to the free option.
Overall you could do with optimizing the site as a single post is making a lot of requests. These tips may help with that:
https://generatepress.com/fastest-wordpress-theme/