There is not just one polygon but many of overlapping polygons, potentially millions.
The GPU can rasterize them in near-zero time even if there are millions, so it is a safe bet.
But it is true that you could also get the silhouette by computing the union of all the polygons. I don’t think there is a VTK filter for it, but you can use shapely or others. It would be interesting to see how much time it takes.