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.

Marcel Pol

Benoit Chantre

Remco

A simple ‘related posts’ plugin that lets you select related posts manually. Supports any post types in WordPress, including custom ones.
Features:
<ul>
<li>Add related posts to your blog posts, pages etc.</li>
<li>Choose from posts, pages or custom post types.</li>
<li>Re-order related posts via drag and drop.</li>
<li>Lightweight in code and database-requests.</li>
<li>Includes Widget that shows the related posts.</li>
<li>List of posts can also be added to the content of your post.</li>
<li>Custom markup possible, build your own caroussel or anything you fancy.</li>
<li>Support for multilanguage plugins, like WPML and Qtranslate-X.</li>
<li>Duplicate plugin Related (Doubled Up) is included to build a second list.</li>
</ul>
The plugin was written to have the option to add related posts to each blog post using a simple but functional plugin. You can select the related posts yourself manually.
To display the related posts, there are three options:
<ul>
<li>You can use the widget that is included.</li>
<li>Use the content filter inside the settings.</li>
<li>Add PHP code to your template, see the installation docs.</li>
</ul>
For advanced options, see the installation docs.
<h4>Compatibility</h4>
This plugin is compatible with <a href="https://www.classicpress.net" rel="nofollow ugc">ClassicPress</a>.
<h4>Contributions</h4>
This plugin is also available in <a href="https://codeberg.org/cyclotouriste/related" rel="nofollow ugc">Codeberg</a>.

Manual Related Posts

Does exactly what is says it will. Very easy to integrate where you need it. Kudos to the dev who made it.

Simple and Easy to integrate

Very handy plugin for manually set your related posts !

Simple and Efficient

A simple 'related posts' plugin that lets you select related posts manually.

Efficient Related Posts vs Manual Related Posts