Thank you for the reply Mylenium.
I emptied and disabled all caches in CS6 and the render speed did improve, but not much.
Just finished rendering the same Comp and the render time is now 27 Min 0 sec, an improvement of 2 Min 4 sec but still more than 8 x longer than CS5.5.
So my question stands. I've read elsewhere that expressions aren't processed well in CS6 and CC, could this be the reason for the dramatic discrepancy in render times?
There are any numbers of issues with temporal expressions due to the cache stuff, but normal linear math expressions should evaluate properly. anyway, you would have to provide the project to verify that. also check for otehr things like specific footage types that may not work the same across versions. E.g. the MFX loaders pretty much got updated in every version of AE, not always for the better...
There are 2 temporal expressions in the Comp, the fade out is applied to 2 layers, the text expression selector to 1 layer.
The video BG is an uncompressed render from After Effects using the Animation Codec, its 1.3 GB, 3 smaller rendered Animation Codec QT's total 180 MB. All other layers are .psd files (total < 10MB per comp), and 6 shape layers.
As an experiment I converted all expressions to keyframes – though the AmorType did not convert properly.
CS6 rendered the file in 2 min 16 sec more than 10x faster than with expressions.
I'll continue to experiment to determine which expression(s) cause the glacial render slowdown.
Many thanks for your input.