This is one of those "don't sweat it.... maybe" questions. Once AE knows a layer's opacity is zero, it may NOT do anything with it.... unless it's part of an expression elsewhere in the project. The sure-fire way to keep AE from evaluating anything is to trim up in and out points. Then there can be no question about it.
Thanks Dave. I probably should of evaluated a little more on what I'm doing exactly.
I'm building a character rig for animators who aren't quite... well.. animators, long story.
Anyway, they don't know how to dress this rig up or navigate all the pre-comps for the various projects they have to work on, so making the character customizable was a necessity (different hair, different eyes, different noses, etc)
I have the many different layers controlled by checkbox switches in the Master Comp, which work great.
Problem is, as time has gone on, and I'm being asked to add more and more different options for layers the rig is really starting to slow things down. So I'm in the process of optimizing it as best I can.
I asked about opacity because right now the different layers have their opacity switch between 0 and 100 depending on which ones the pseudo-animators want to use.
So they do have expressions on them, all of them in fact. Now I'm starting to realize where the slow-down is coming from...
Thanks again Dave!
The issue are the expressions themselves, not so much the layer opacity. AE's expression performance is just terrible, especially when evaluated across comps. You're basically telling AE to check the expression for every temporal sample (frame or sub-frame, if motion blur is used), which could easily mean several hundred look-ups every time. Otherwise any pixel that has a value of exactly zero will be left out of any evaluations, give or take certain features and effects that may use invisible layers internally or evaluate visibility of layers at different times.