I've been searching around different forums looking for an answer, a chart would be even better. I'd love to know which effects load down your comps the most. For instance Fractal Noise vs Turbulent Noise. I know turbulent is a little lighter on your CPU. Maybe this wouldn't affect one instance, but when you've got 30 or more clouds in precomps it can slow you down a bit. Even just a list of really light on the cpu effects would be rad.
More specifically I'd love to know:
-Levels vs Curves vs Exposure
-Direction blur vs Radial blur
-Camera Lens blur vs RSMB
-Set Matte Vs Track Matte
-PNG w Alpha vs Separate Alpha luma matte
Profiling the render time on a project is fiendishly tricky problem: it looks like it should be simpler than it is. Camera Lens Blur is very "computationally expensive" (read: slow to render), but just as an example of how difficult your question is to answer directly, HOW slow it renders depends on the blur radius and iris shape.