The only reason to trace one ray at a time is to visualize what the integration is doing. To trace many rays at once, close the Detector Control, then change the number of analysis rays on the source from 1 to 500 in the NSC Editor:

Now reopen the Detector Control, and press “Clear Detectors” then “Trace”. All 500 rays will be traced at once, and the resulting PSF will be displayed in the detector viewer window.

Even though the rays are randomly selected the PSF converges to the correct Airy pattern (this particular lens is diffraction limited).