You say big scenes ... do you mean a big picture (Pixelwise) or a big scene with many items?
Because if it is just pixelsize big, you don’t need that much RAM.
RAM is holding the textures and so on - if there aren’t many textures, it doesn’t need much RAM.
The rendertime depends on your CPU, and that sounds fine.
Did you look in taskmanager whether all cores are more or less working when rendering?
If no, you could take a look whether it is running at a lower priority.
Edited again: and are you sure it’s not running with some compatibility mode set by Windows?
Somebody had a problem with DazStudio being slooooow and found out his desktop icon somehow was set to Vista compatibility ... for whatever reason.