it is more than a year that I haven't used jackrabbit, at that time there was a component named "Persistence Manager" responsible for mapping nodes and it was just a simple one, not much optimized, now there are more Persistence Managers developed and they may be as fast as CRX and other commercial implementations.

Alfresco has developed a test suite for JCR implementations, you can run the test suit against different JCR implementations and pick you choice try the followings:

1- Alfresco
2- Oracle
3- EXO
4- CRX