Texture down-resing

A day or two ago, I down-res'd all of my textures for the exterior shot to the bare minimum dimensions needed for each camera angle. Expecting a huge cut in render time, since I was going from 2x4k down to 512x1024 in some cases, and even from 2048x2048 to 128x128, I was disappointed to see Renderman drop off a mere 12 minutes from what was originally a 104-minute slumberfest. I'm trying some other options right now, namely converting images to .tex, which RmF does as part of its rendering process. I'm also to try MIPMAPing with Mental Ray, which will recursively reduce my texture files automatically according to the camera's distance and resolution. Hopefully I'll write again soon with better news.