Apache Lucene 2.9.0 is released. Performance improvements were a large part of this release. Also, Lucene has improved over the way it handles numbers (smaller, greater, between) as previous implementations were mostly text based and these operations were often implemented inefficiently or were done by using custom encoders. 

Many other changes and improvements can be found here.  As well, this release is not fully backwards compatible, so check out the CHANGES.txt document which indicates breaks in the compatibility.