EtherCAT topology and timing constraint question
EtherCAT networks can have different topologies: line, tree, or star.  During start-up, the network topology is determined so that devices know which ports to pass the data through.

For synchronous operations, the master device initiates sending the packet frame through the network.  The slave devices read or write to the bits that they have been allocated.  The bit locations are pre-determined during network configuration.  This is done "on-the-fly" as the frame is passed along to the next device.

The amount of processing performed by the slave device when passing the packet through is minimal.  Often just reading/writing bits to/from a buffer or register.  More complex "processing" is handled across multiple cycles.

For example, a servo drive that is configured to take a position command may take many seconds to control cycles to complete the trajectory.  The drive will report its "not-done" status in each EtherCAT packet until it has reached the target position. 

Review the references on the Wikipedia page for more details.