Simulation speed when many tracks in one event

I’m simulating electrons propagation in a vacuum chamber. My userAction is simple, and it kills electrons when them hit the chamber. The simulation time is recorded from beginofEvent to endofEvent. A strange thing is that when I put 10000 tracks in one event, the simulation time per track exceeds 10 seconds per event. But if I simulate 10000 events with one track in each event, the simulation time per track is less than 0.1 seconds. The test is performed without multi-threading.

Do you have any idea what’s the reason is? Thanks

Best regards,