It depends. Not so much render time directly as the amount of memory used.

What I tend to do is if I expect to use the character again (and not just a one-off), I'll certainly save the face as a morph. If the characters is going to be nude, wearing dynamic clothing, or a second skin-style outfit, I'll save the entire character as an FBM and load it into a blank character.

If the character will be wearing conforming clothing (and this is pre-PP2014 and the fitting room), I'll only load the specific morphs I need that will effect the clothing, and all the other morphs can be part of a FBM. For example, for a female character wearing conforming clothing (but who might also appear nude), the FBM might consist of the morphed face, long fingernails, a modified naval, and genital slit. The other morphs I need to shape the body I'll load individually so the conforming clothing will work properly.

What this means in practice is that I might have two CR2s of the same character: one where the whole thing is a FBM, used for when the character is naked (or in a second skin), the other where the FBM is only those face and minor morphs, and the rest are individual.

You save a ton of size in the CR2 that way, which means more memory available.

If you want to save even more space, go into the clothing and delete the morphs you don't need there as well.