mitcho (Michael Yoshitaka Erlewine) on "[Plugin: FeedWordPress] Related Posts plugin and FeedWordpress (= large blogs)"

@niska YARPP has no inherent limit on the number of posts, and I have heard of blogs successfully using YARPP on installations that large. Just make sure to turn off the "consider tags", and "consider categories" options, not choose any "exclude tags/categories" options, and turn off "cross-relate posts/tags". Those options will make YARPP as lean as it can be.