Any suggestions on identifying root cause as to why the adding of any nodes back to the picture slows things way down? We have an SR opened with Oracle Support but nothing firm so far.
Without having any details on the application that runs on the nodes and their configuration, one can only guess.
But generally speaking, I'd suggest checking what specific business operation(s) slow down the most when adding another node, doing extended SQL traces on their sessions (or single statements that show performance degradation) accordingly and checking where the time gets lost.
Best regards ... Ralph Graulich ... -- http://www.freelists.org/webpage/oracle-l