RC2 contains numerous bug fixes, enhancements and a wide range of performance improvements. The new range index - introduced with RC1 - has seen a lot of real-world testing and bug fixing to make sure it is properly used by queries, so they benefit from the superior performance of this new index.
Check the announcement to learn more.
RC2 is a last test before 2.2 will be released as the next stable version of eXist.