As a product manager for YARPP, I really appreciate the mention here, Jeff. Just wanted to clarify:

There are two versions of YARPP. It's true that YARPP Basic (free) can take up resources on a site owner’s webhosting server due to the way it stores and indexes recommendations. However, YARPP Pro (also free) runs off of our servers, so it does not have the same resource burden for site owners. YARPP Pro is a better for larger sites.