Round-trip time is that plus the average time for a response to get back. Delay is one of the IP performance metrics . Network delay is often said to be composed of propagation, serialization, and queuing delay.
Propagation delay is the time it takes the physical signal to traverse the path.
Serialization delay is the time it takes to actually transmit the packet. In modern world where high bit-rate connections are becoming the norm serialization delay is becoming more and more irrelevant. Queuing delay is the time a packet spends in router queues. It is the most variable delay component in a typical modern network. In this case it evidently cannot be significantly lessened due to fundamental physical reasons if physical network configuration is such that packets take almost the straight line as their path.