I shall try to explain my self as accurate as possible.
Say I have 3 layers in a composition. They are stacked on top of each other, and only the bottom layers is visible at the start (the other two layers are keyframed on by opacity later).
I have a very ram/processor demanding effect (Camera Lensblur with 50px blur) on the bottom layer, and none demanding effects on the other two.
My question is now:
Does After Effects take the bottom layer in to account, even though it has been obscured by the non-demanding layers on top, or does AE only "look at" visible layers?
Thank you in advance
This depends on the version of AE you're using. The newer versions do a better job of optimizing renders.
One way to find out how well this works is to test it. Do a render and see if it speeds up significantly once the bottom layer is totally obscured.
I've been using AE for over a decade, so I'm used to trimming layers once they're no longer visible in order to make renders faster.
Yup. Trimming up layers is good.
Not only does it keep your comp tidy, you get a better idea of what's happening in the comp just by looking at layers.