Due to the reliable group communication platform, network delay is amplified by the necessary acknowledgments between the group members. The speed of light puts a lower bound on the minimum network delay. For example, a laser pulse that travels through a fiber takes ms between New York and San Francisco, ms between Paris and San Francisco, and ms from London to Sydney. In practice the networks today are slower than the lower bound by about a factor of (due to switching overhead, etc.).
To put this into perspective, an 850MHz Pentium III PC performs a single 512-bit modular exponentiation (one of the most expensive, but most basic public key primitives) in under 1 ms. Moreover, the speed of computers continue to increase. Comparing this with the WAN network delay, it is clear that reducing the number of communication rounds is much more important in the long run for an efficient group key agreement scheme than reducing the computation overhead.