An app note (PDF!) from Texas Instruments on importance of latency in factory automation:
Latency is a critical parameter in Ethernet networks developed for Factory Automation applications. Latency is not a defined value for Ethernet as specified by the IEEE 802.3 standard. Nor is Ethernet inherently synchronous or repeatable. This disconnect between the inherent characteristics of Ethernet and the needs of Factory Automation applications must be bridged though careful architecture of Ethernet Physical Layer devices like the DP83867.
The latency through the Ethernet Physical Layer is a key limiting factor of control cycle time in these applications. The capability of the DP83867 to operate with less than 400ns latency in 1000Base-T operation can improve the cycle time. This low level of latency is also comparable to levels seen in 100Base-TX operation, thereby simplifying the process of upgrading a 100Base-TX network to 1000BaseT and increasing the network bandwidth.