To set the cycle time of the virual PLC in an IEC 61131-3 task, first stop the task and right click the task name in the control panel.

Then click on the Execution Mode tab.

The fastest cycle time that can be set is 1 msec if the IEC task is run on a FAST process.  Note that making the SERVO_PERIOD faster does not increase the available cycle time and might make the PLC task overflow due to more processor time being allocated to the motion.