Related posts in your blog is a good way of increasing page views and the time users spend on your blog. The problem with many related posts plugins is that they slow down the blog considerably (though popular posts plugins are worse). This is because many of them create the related posts list for the posts every time a post is loaded. Some supposedly support cache but it doesn’t seem to help.
I came across a neat related posts plugin, the Efficient Related Posts. It uses a more intelligent approach as it generates the related posts whenever a post is saved. This is a much better approach as posts are viewed more times than they are saved. It knocked off 0.40 seconds in the loading times of the one of the sites we maintain.
It also supports the normal options like categories to ignore, number of related posts to display and where to insert the related posts (single posts, pages and RSS feed). Try it out and let us know in the comments how it goes.