Michael Gronager has figured out why:
I now exit the ipc thread at startup by inserting:
void ipcThread(void* parg) { ipcShutdown(); return; ...
Bitcoin-Qt is now running nicely using around 0.9% CPU. So it seems like the culprit was indeed line 31:
if(mq->timed_receive(&strBuf, sizeof(strBuf), nSize, nPriority, d))
More research into what's going on:
boost::interprocess is busy-waiting (at low priority), because it is emulating Posix functionality. See the comment is in boost/interprocess/detail/workaround.hpp : //Mac Os X >= Leopard defines _POSIX_THREAD_PROCESS_SHARED but does not seems to work.
... or google for "osx _POSIX_THREAD_PROCESS_SHARED"
I'm still thinking about the best way to workaround this; having Bitcoin-Qt use 100% cpu whenever running will trigger too many bug reports to release it the way it is.