That mesh has half a million faces! (469 K) It’s also far from the origin, which can cause problems.
You can sample it at lower resolution, if that’s what you want? Many examples on this forum.
MeshToSurface_2024Feb5b.gh (7.1 MB)
But why extrude it? You can project the edges to a plane to create sides and bottom, making an enclosed volume with the terrain on top. Mesh will be faster than NURBS though.
MeshToSurface_2024Feb5a.gh (6.4 KB)
(without mesh)