A "round trip" is the amount of time it takes to send a message to an object and receive a response.  This time is dependent upon the speed of the channel used to send the message.  For example, sending messages over SharedMemory is very fast.  Sending messages over the Internet is relatively slow.

See GestaltRoundTrip
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By all means think of the '''Round Trip''' time, but it comes in two parts. Most people only think about the speed of the link, but most of the time it's the latency that kills you. http://www.stuartcheshire.org/rants/Latency.html