You're using Poser 5, right? I don't know how it effects the rendering times, but instead of modelling with polygons, you can make a strip of sidewalk as a single box, and make the joints between the slabs with a displacement map. Check for Geep's Magic Box. And you can use the Grouping tool to copy that without the ground-facing polygon. Why use 6 surfaces when you can't see one of them? I've not noticed a rendering slow-down from tiling a texture. And something I haven't tried, but maybe you can feed two seamless texturemaps into the two different tile-types on the Tile shader, with zero mortar thickness...