Increasing the Threshold value will have dramatic effects on the time it takes to process and image. Higher thresholds mean significantly longer times to process. The relationship is not linear.
Increases in Flare Intensity and Spike Points will also significantly increase processing time, although probably not as much so as the increase in Threshold.
Changing the Spike Color from white to either background or foreground will increase the time to process the image, but the increase is not significant. The same is true for decreases in Flare Opacity.
Changes in the other values will have little to no effect on processing time.