If you're suspicious of the event loop, could overriding QCoreApplication::notify() and dosome manual profiling (one or two maps of senders/events to counts/time)?
I'm thinking that you first log the frequency of event types, then examine those events more carefully (which object sends it, what does it contain, etc). Signals across threads are queued implicitly, so they end up in the event loop (as well explicit queued connections too, obviously).
We've done it to trap and report exceptions in our event handlers, so really, every event goes through there.
Just an idea.
