Thanks to Andreas's feedback, I've been able to narrow it down further:

it looks like getNextEvent() is not beeing called nearly as  
frequently as it should during event-idleness, only about once every  
3-10 seconds instead of once every 1/2 seconds as Andreas indicated.   
The code that does this is #ioProcess in EventSensor, which waits on  
the I/O semaphore for EventFrequency milliseconds (actually,  
EventFrequency defaults to 20 ms, so we should be checking 50 times a  
second).

It is this wait for the Semaphore to time-out that is taking much,  
much longer than expected, up to 10 seconds and more.

So now to track that down...

Marcel