"First, you should also check out WPDB Profiling. Second, we used to use YARPP, but with an investor site that we did which had 6000+ posts and 1800+ tags, it completely choked. Even WP-Related-Posts would cause pages that displayed related posts to take 10+ seconds to load. We ended up having to write our own plugin (which is not released yet, but I hope to eventually). We called it "Related Posts -HV" (for high-volume). Basically, it stores related posts as post meta when a post is saved. It moves the heavy lifting to a one-time thing that affects the post writer, rather than affecting everyone that visits the site. However, you don't want ONLY older posts to show as related, so you need to re-process a lot of posts when a new one is saved. We created some logic that gives a value of HOW RELATED two posts are. When a post is saved, that value is generated for all posts (comparing the new one to all existing posts). The top X (you can set this value) are saved as post meta. However, every post with a r" - Jeroen De Miranda from Bookmarklet

There are some really good tips in here. - Mike Showalter