X ref scene renders slow

[fareen]

Hi out there

 

I am rendering a huge aerial shot in which everything in the scene is x refed from diffrent files.the problem is that the task manager shows a max of 13% being used while in other cases(i.e when there is no x ref scene) it shows 100%.Although the memory used by it is high.Is this a v ray thing where the x ref scene does not take up the full resources of the comp? Or do i have to do certain settings to my comp. Its a 64 bit Quad core 4gb.

 

also is there any way where i can speed up the transforming vertices part of the rendering?