You change the screen on the OP by pressing a button to set a bit in the PLC. Depending on the status of this bit the PLC job screen change is executed or not. This procedure takes a relative long time, because the interface area must be polled first in order to recognize that PLC job is to be executed.
If you reduce the basic cycle in ProTool, you shorten the response time considerably (the interface area is polled more frequently).
At the same time by reducing the basic cycle you also shorten the polling time of all variables / pointer areas and so increase the communication load unnecessarily. You avoid this by increasing the polling time of the other variables accordingly in order to restore the original communication load.