One of the new things I'll be working into Roller as part of the next batch of performance improvements will be the ability to process referrer information in an asynchronous manner, i.e. outside the http request/response cycle. Not of much importance to smaller Roller installations, but enterprise users should get a pretty nice performance boost. The official proposal page is here ...

http://rollerweblogger.org/wiki/Wiki.jsp?page=AsynchronousReferrerProcessing