Planet textures doesn't contain mipmaps by default and D3D9Client will create mipmaps on the fly and it is the DXT compression phase that will slowdown loading process. You can disable that by setting "Tile mipmap policy" to "none" from the D3D9 settings on a launchpad. Mipmaps do improve rendering quality a lot, you can observe Enceladus with and without mipmaps to see the difference.