We have not experienced this kind of slowdown.
We need more information to find the reason of the problem, please give us the following information:
Can we see your site? What is the URL of it?
How many content do you have? We have a site that contains nearly 6000 blog posts and products, and this site has a normal speed without any problem.
Do you have more posts and other content than this?
What kind of post types do you have?
Exactly what is slow? Front-end or back-end pages or both? Only those pages on which related contents are displayed or other pages too?
YARPP is the background engine of this plugin. If you deactivate WRI, and use YARPP only to offer related content, does the problem still occur?
Please try if there is a plugin collusion. If you deactivate all plugins but WRI and YARPP, and set a well known theme (e.g. Twenty Fifteen), the problem still occurs?
You can also answer to firstname.lastname@example.org, if the asked information is not public.