Set the "Multiple displays" option to "Extend these displays". It has been reported that some monitors (especially the ones designed for vision science, such as ViewPixx, Display++, etc.) should be the main display of the system, to pass the latency test and use refresh rates recommended by manufacturers.
After changing the Display settings, restart NIMH ML so that the changes are updated.
Run the latency test from the Video menu. The red lines in the Movie Display Results indicate the start times of new frames. See if they show up regularly as in the example below. The intervals between the lines should be the same as a frame length (e.g., 16.67 ms for a 60-Hz display).