This is a twofold question.

1. What is the best order to put adjustment layers in?

2. Are adjustment layers slower than the "old" way?

More details

1. Stacking Order

After noticing some irregularities in my render times, I have discovered that it matters A LOT which order you apply effects in adjustment layers--how they are stacked.

For example, I have a simple sequence with 30 CamA/CamB cuts. I want apply an RGB Curves color correction ("CC" below) and Neat Video Noise removal effects to the entire sequence, so I do it in the form of the new adjustment layers--one for CC and the other for Neat.

Here are my observations and render times of the exact same sequence with different stacking orders of the exact same effects:

1. 0:43  CC + Neat
2. 2:30  Neat + CC
3. 0:14  No Effects
4. 0:15  CC only
5. 0:42  Neat only
6. 0:30  Neat only - Effects applied to each individual cut with no adjustment layers

I haven't done extensive research on with other effects (I'm hoping someone else, or especially Adobe has). But I couldn't figure out why two similar sequences would take 2 hours and another just 30 minutes since they were very similar and I noticed that my adjustment layers were in different order.

QUESTION

It's obvious, at least from this limited test above, that the stacking order of effects is important. Only now, after doing the testing, I know that I need to do my CC first THEN my noise removal. But what about with other effects?

2. Adjustment Layers Slower?

From the above testing, I noticed another irregularity (IMO). In my test result #6 above, when I applied the exact same noise removal effect to each individual clip as opposed to an adjustment layer, my render time was reduced by 33%. 

Does using adjustment layers present a render hit when compared to using effects the "old" way (clip-by-clip basis)?