Face set problems! why so slow?

I try to use inventor to render a height mesh with set of faces(about
20,000  quads)

but the inventor initlizes about 2 mins$B!$(B I think it should not be
so
slow.
I post the code, and hope someone can help me. Thanks

	SoSeparator *pHeightMapSep = new SoSeparator;
	root->addChild(pHeightMapSep);

	float x, y, z;

	SoBaseColor *pHeightMapColor = new SoBaseColor;

	SoCoordinate3 *pHeightMapCoord = new SoCoordinate3;
	pHeightMapSep->addChild(pHeightMapCoord);

	int numofx = 399;
	int numofy = 499;

	vector<int> iVecNum((numofx-1)*(numofy-1),4);
//	allocate arrays {4,4,4,4,4,......}

	int iIndex = 0;

	for (int i = 0;i < numofy-1;i++)
	{
		for (int j = 0;j < numofx-1;j++)
		{
			x = rp[(j+i*numofx)*3];
			y = rp[(j+i*numofx)*3+1];
			z = rp[(j+i*numofx)*3+2];
			pHeightMapColor->rgb.set1Value(iIndex,0.8,0.8,0.8);
			pHeightMapCoord->point.set1Value(iIndex++,x,y,z);


			x = rp[(j+i*numofx+1)*3];
			y = rp[(j+i*numofx+1)*3+1];
			z = rp[(j+i*numofx+1)*3+2];
			pHeightMapColor->rgb.set1Value(iIndex,0.8,0.8,0.8);
			pHeightMapCoord->point.set1Value(iIndex++,x,y,z);

			x = rp[(j+(i+1)*numofx+1)*3];
			y = rp[(j+(i+1)*numofx+1)*3+1];
			z = rp[(j+(i+1)*numofx+1)*3+2];
			pHeightMapColor->rgb.set1Value(iIndex,0.8,0.8,0.8);
			pHeightMapCoord->point.set1Value(iIndex++,x,y,z);

			x = rp[(j+(i+1)*numofx)*3];
			y = rp[(j+(i+1)*numofx)*3+1];
			z = rp[(j+(i+1)*numofx)*3+2];
			pHeightMapColor->rgb.set1Value(iIndex,0.8,0.8,0.8);
			pHeightMapCoord->point.set1Value(iIndex++,x,y,z);
		}
	}

	SoFaceSet *pHeightMapFaceSet = new SoFaceSet;
	pHeightMapFaceSet->numVertices.setValues(0,iIndex/4,(int
*)&iVecNum[0]);
	pHeightMapSep->addChild(pHeightMapFaceSet);