PCL filter order

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In almost all cases, it will be faster to run CropBox first, because it is very fast and can potentially eliminate a lot of points. How large the advantage is depends on your data: If you have a huge point cloud and a small target volume, so that CropBox removes 90% of points, the advantage will be greater than if CropBox only removes 10% of points.

That being said, VoxelGrid is quite fast too, so I suspect in most cases it wouldn't hurt too much to run VoxelGrid first (but there should almost never be an advantage to this).