bgtwuk, loop time is set with the hardware OS.ExecTimeOut parameter. Have a look at page 10 of the DP610TM API doc, then set up the Service Tool to monitor ExecTime, ExecTimeWork, ExecTimeAppl, and ExecTimeGraph. It will give you a good understanding of what the screen is busy doing. You may be surprised to find how much time is spent updating the screen versus any i/o, which is important if you connected any expansion i/o to the display.
I usually make a NV parameter for setting the ExecTimeOut, and put it in the OS Outputs page, as shown in the attached image.
Play with ExecTimeOut and watch what happens with the measured Exec* parameters.