Thank you all for your assistance. I believe I've found the root cause if the behavior I am seeing.
If I just use "SELECT * FROM MyEventTable" (Fabian's question), I find that events received in the first 3 seconds are ignored as opposed to the original 5. What I'm seeing seems to suggest that the system is just not ready to start receiving events during that time, i.e. the underlying DataStream is still getting created, and that removing the GROUP BY reduces the overall size of the stream which in turn reduces the initialization time. Many thanks, John
