I just completed (for the most part) a database optimization which was slowing the site down.
In a nutshell, when I created some new code to mitigate against Google soft 404 "errors" (slim content) by adding matching man pages to posts with very little content, I used a "real time" FULLTEXT search of over 350K man pages, and this was causing some performance issues.
Plus, my new code worked, and Google search referrals have just about doubled since I created the solution, but the upside/downside was that the site traffic also doubled (which is a good thing) and the new queries were killing performance (a bad thing).
In the final optimization, I preprocessed all these matches and stored the results in the DB (the ids of all the man pages which match the discussion contents) so, the long MySQL queries doing these FULLTEXT NATURAL LANGUAGE searches on the fly were eliminated (right now about 99.5+ are gone, I have about a half a day of processing to get to 99.999 percent.
The performance improvement is remarkable and the site is much faster, and I have earned my black belt (or at least a brown belt) in MySQL performance tuning, after all these years.