A short time ago I was asked to checkout the performance of an 4 node 64 bit linux RAC cluster on 188.8.131.52 RDBMS. After a short investigation I found out that the system was waiting for a very rare wait event, called: "ksu process alloc latch yield". This wait event was comming up on all nodes (statspack and current session waits).
Looking at Metalink the issue is only mentioned as part of bug 4658188 and the wait event was introduced in 184.108.40.206. The problem is only occurring in combination with RAC, because according the bug LMON (Lock Monitor) is waiting on the event. I was able to make sure, by mapping the session waiting on the event, to the LMON process (shows up as ora_lmon_<name>).
Because of the bug the LMON process is slow and in that way the whole RAC cluster. For this issue a one of patch is available on top of 220.127.116.11 (one-off patch for Bug 4658188).