The W-term increases for lower elevations of the target field. Using the W-Projection algorithm, the computing time increases with w². This figure shows the normalized cumulative distribution of the w-coordinate (left panel) for a typical LOFAR observation, and the corresponding normalized cumulative computing time (right panel). We can see that rejecting the  ~5% of the points with w > 10⁴ saves  ~70% of the computing time.