Purge your RAM before rendering.
Only show one comp window. You know, so you can see the contents. Other comps, whether tabbed or not, don’t affect rendering or previewing. I hope that’s clear …
Pre-rendering layers that require processing does save time. If the greenscreen actors don’t require significant processing, you won’t save time. Regarding HDV and Cineform, I can’t speak about Cineform, since I don’t know it. But HDV should not be used in AE, as it bogs down the system and can cause errors.
Prerendering a layer makes it into a still or a movie. Keep in mind that a layer is really a flat plane, so it never was 3D in the first place. If it was “AE-3D”, meaning that it was a flat layer recognized by AE’s 3D space, then a prerendered movie can behave the same way as a layer if you import it and click its 3D button. However, if you pre-render a comp, its layers will be flattened into one layer: a movie. If that matters.