How much time FCM takes to deliver push notification to a topic of large base

We are planning to implement Topic Messaging via FCM to send PN to our base, I want to know how much time FCM takes to deliver PN to apps via topic:

1. with 10 million subscribed to a topic
2. with 50 million subscribed to a topic
3. with 100 million subscribed to a topic

I understand delivery will depend on apps being online, however we can assume here hypothetical case of every one being online. In other words, I want to understand how does FCM processes sending PN to a topic and how much delay it will/might introduce?

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