In general, performance is a measure of the speed and efficiency with which a message service delivers messages from producer to consumer. However, there are several different aspects of performance that might be important to you, depending on your needs.
The number of message producers, or message consumers, or the number of concurrent connections a system can support.
The number of messages or message bytes that can be pumped through a messaging system per second.
The time it takes a particular message to be delivered from message producer to message consumer.
The overall availability of the message service or how gracefully it degrades in cases of heavy load or failure.
The efficiency of message delivery; a measure of message throughput in relation to the computing resources employed.
These different aspects of performance are generally interrelated. If message throughput is high, that means messages are less likely to be backlogged in the broker, and as a result, latency should be low (a single message can be delivered very quickly). However, latency can depend on many factors: the speed of communication links, broker processing speed, and client processing speed, to name a few.
In any case, there are several different aspects of performance. Which of them are most important to you generally depends on the requirements of a particular application.