there is one pdf linked in here somewhere about vue6 render settings ... you would have to use search or maybe someone has the link at hand ...

from what i have heard the predefined render settings (final, broadcast etc.)  are not balanced very well and there are better ways (described for vue6 in the above mentioned pdf). i think the author is working on a vue 7 version as the settings have changed a little.

and the obvious factors that slow down rendering speed are better lighting models (AO, GI, GR and volume/spectral lighting), transparency, mirroring, volumetrics, antialiasing, larger image sizes, complex materials (check the function editor to see if there is much going on there)

thats about all i know :)