Although I am not familiar with emqx, from their web page I can see they have a more "enterprise" solution and integrations with Oracle, Timeseries database, Apache Kafka. With QoS 0 they say that message throughput is around 200,000/s while on QoS 1 it's around 100,000/s. If broadcasting, they manage to get around 900,000/s. They've got a ton of features and I'd say probably a better, but paid solution compared to some other brokers.