Effectiveness of Cloud Features for Firebase

I am interested in evaluating the efficiency of using Cloud functions for Firebase, so I installed a simple function with the same code as this example from Firebase . What the function does is simply listen for new messages on a specific node in Firebase and then send a push notification to the recipient of that message.

Then I wrote a script that bombards the node messages, adding x new children there every second. I found out that the function seems to be capable of handling up to 40 new messages per second, whatever, and notifications are starting to lag more and more. This seems like a pretty bad job - too bad for us to use in production - and I'm wondering what bottleneck there might be. Of course, the functions are still in beta, but according to the documentation , each function should be able to run 400 concurrent calls.