Monochrome film also has certain differences in colour response. Even modern panchromatic films are less sensitive to the red end of the spectrum, while the older films hardly responded at all. So if you want a look more akin to the silent-movie era, you can nearly throw away the red channel of the image. That also led to rather odd-looking make-up styles. For rendering speed, altering a texture in the shader tree is going to slow things down. You need to pre-process the texture maps as much as possible. Photographers routinely used filters to cut back on blue light, but you wouldn't really need to do this in Poser, since you don't have the extra sensitivity. White clouds against blue sky would vanish without this being done. Bearing all that in mind, have a look at some movies from the time you want to copy.