but the morph data for a single morph for a single body part is what, a few hundred kb?  very minimal.  add transparency to a scene with one raytraced light, and i've seen render time on small test images go from minutes to hours.  it's why i test without hair when i can. 

i don't know if the renderer cares what how the transparency is controlled, or if it's just a matter of what percentage of the screen the transparent object takes up.  but in general,   the moment i introduce transparency into my renders, it can take me days just to make tweaks because of how long tests take to do test renders.