Aaron D. Campbell

There is a <a href="http://bluedogwebservices.com/problem-related-post-plugins/" rel="nofollow ugc">problem 
with related posts plugins</a>, and Efficient Related Posts is fixing that by 
approaching the problem from a different direction and offering a very different 
solution.
Basically, current related post plugins build the list of related posts on the 
fly when the user needs to view it. Since blogs tend to be viewed far more 
often than they are updated (often hundreds of times more often), these queries 
are run way more times than they need to be. This not only wastes CPU cycles, 
but if the queries are slow (which they will be if you have 1000s of posts and 
tags) then the user gets a poor experience from slow page loads.
Efficient Related Posts moves all this effort into the admin section, finding 
related posts when a post is saved rather than when the user views it. The 
advantage is that if the query is slow it happens less often and the post writer 
is the one that waits rather than the user (which I think is WAY better).
There are limitations. For example, since the related posts are stored as post 
meta data, we only store a certain number of them (10 by default, but you can 
set it to whatever you want). This means that if you decide you need to display 
more than 10, you need to have the plugin re-process all posts. I generally 
display up to 5 related posts, but store 10 just in case I decide to display 
more in some areas. Also, since the related posts are calculated when a post is 
saved, manually adding a tag through the database will have no effect on the 
related posts, although I recommend not doing that anyway.

Efficient Related Posts

A related posts plugin that works quickly even with thousands of posts and tags.  Can be added automatically to the end of posts.

WebshopLogic

WP Related Posts and Products plugin offers different types of similar contents for your users. For example, some related products can be displayed to your similar posts. This makes connections of your site contents visible to your clients, increasing business efficiency.
Would you like to offer some related products to your blog posts? Do you need to offer up-sell or cross-sell products? Do you have an event calendar plugin, and want to suggest some programs connected to an article? Do you have a custom movie catalog plugin and want to associate some articles to your movies?
Using cross type relationship management functions, WRI makes it possible to associate a post, page, or other custom post type to other posts, pages or custom post types, ensures widely configurable relatedness settings, sophisticated cross-relation adjustments. WRI uses the most popular YARPP relationship handling plugin in the background, extending its functionality, retaining its advantages.
WRI combines automatic, manual and common categorization based relationship management. Some functions are available in the PRO version.
You can set attributes that define how to display different type of related items for every reference item type. Such a matrix-like way you can specify all necessary variations of display settings.
This version ensures built in WooCommerce custom product support.
<h4>More information, documentation</h4>
LITE: <a href="https://webshoplogic.com/products/wordpress_plugins/wp-related-posts-and-products-lite-plugin/#tab-faq&noscroll" rel="nofollow ugc">WP Related Posts and Products Lite Documentation</a> 
PRO: <a href="https://webshoplogic.com/products/wordpress_plugins/wp-related-posts-and-products-pro-plugin/#tab-faq&noscroll" rel="nofollow ugc">WP Related Posts and Products Pro Documentation</a>
<h4>Available languages</h4>
<ul>
<li>English</li>
<li>Spanish – translated by Andrew Kurtis – WebHostingHub </li>
<li>Hungarian – translated by WebshopLogic</li>
</ul>

WP Related Posts and Products (WRI)

WP Related Items plugin offers different types of similar contents for your users. For example, some related products can be displayed to your posts.

Efficient Related Posts vs WP Related Posts and Products (WRI)