CPU usage depends on the effects you are using, the bandwidth of your system, and several other factors. When you have a machine with a bunch of fast cores and a bunch of memory the bottle neck that slows things down may become the read write speed of your hard drives.
You can try different memory and processor allocaions in the preferences file. Here are some resources for the optimum settings.
Yes, some effects and features are less computationally intense than others. There is no reason at all to expect 100% CPU usage in any scenario. This may occur with effects like Trapcode Particular, but not e.g. with a simple blur.
I'm having the same issue. Kind of. When doing a RAM preview with "Render Multiple Frames Simultaneously" on, my cores max out and my fans start roaring. My RAM preview completes very quickly. But, when I save out an actual render, it goes crazy slow, the cores are only used about 5%, and it takes about 5 times as long. Any ideas?
2 x Intel Xeon E5645 @ 2.40 GHz
Nvidia Quadro 4000
To what formats are you rendering? To what locations? What's your harddrive setup? What effects and features are used? Could be a million things from your drives being slow to using compact clip formats that simply require sequential storing of the frames to specific effects not being MP compliant... Anyway, what you see is probably normal, given the circumstances.