Anurag Goel recently carried out a comparitive test of Xapian/Flax and Lucene/Solr. Some interesting results here: it seems Lucene is faster at building indexes, but Xapian is faster and possibly more accurate at searching. We can expect some further speed improvements over the next few months as a new, more compact backend to Xapian is released.
By the way, the article mentions Xappy: this is a Python interface to Xapian that is a major part of our Flax enterprise search platform. You can get Xappy here.