Messages can be published as fast as needed, however maximum throughput will be limited by factors like network bandwidth, server/device CPU/RAM limitations, ISP, to name a few examples. Our recommendation is to publish 10-15 messages/sec per channel.
On the subscribe side, you will see the same limitations so you should consider how many messages a subscriber can consume based on the use case. If it is messages that a human needs to read then how many messages is too many? If it is data that is driving a realtime graph, then consider how many per second is enough to provide the necessary graphing experience.
For further guidance, please contact PubNub Support and we will assist with best practices to implement high message rates to satisfy your requirements.
See, also: Soft & Hard Limitations