Panasonic A6B EtherCAT servo drives have a ramp down to stop feature that is enabled by default. This means when the Master sets the control word to $06 (Disable) the drive will ramp down the speed to zero and only then disable the drive.  This takes a few seconds.

The EtherCAT parameter related to deceleration is 6084h (Profile deceleration) and this must be set for imediate disable without the ramp-down.