If you have poor database performance and many posts, YARPP will destroy your server.

I rewrote it in-house so it never does just-in-time indexes and instead I allow it to only index when the post is first published and never again.

Then I wrote a command line tool to slowly index the database (for imported posts) and pause based on server load so it only does a few at a time and eventually indexes the whole site over many hours transparently.

The problem is YARPP does complex full-text queries and then a massive unindexed query that has to scan the whole table, for every post. So if you have many posts and a weak server, it will really destroy performance.

Note that YARPP has http://wordpress.org/plugins/yarpp-experiments/ which can help pre-index your posts and you can run it at off hours (it does all those bad queries at once) but it's also very flawed.