Imac Performance Issue with games in windows

Hi all Mac heads!

I would appreciate if everyone could get together att help me with what seems to be the biggest unknown problem in cyberspace.

I own a

Imac 27" 3.5 GHz Intel Core i7
16GB 1600 MHz DDR3
NVIDIA GeForce GTX 780M 4096 MB
OS X 10.9.3 (13D65)
and of cause Bootcamp.
with 149GB of free HDD space (for Bootcamp Partition)
100 Mbit/s Fiber optic internet connection

I have installed Windows 8.1 Pro, (from a student site with a large discount)

and now to the problem. I bought Wolfenstein The new order on a disc (not download)

and the requirements for the game are such as
Minimum System Requirements:
64-bit windows 7/Windows 8; Intel Core i7 or Equivalent AMD, 4GB
System RAM; 50 GB free HDD space; GeForce 460, ATI Radeon HD
6850; High Speed Internet Connection, Steam Account.

and I CANT PLAY IT!
I get these Horrible FPS Drop in game that the game is totally unplayable.
and when I check Task Manager, my CPU activity its up to 0.74GHZ on all processors !!!!

I tried playing it on my girlfriends Macbook Pro (equal, maxed out components) and there I could play it with no problem.

several games like watch dogs and Assassin Creed black flag and Need for speed Rivals have some strange FPS drops but playable, (Exept NFS Rivals)

I have also followed all suggestions to change settings in Nvidia Controll Panal and lowering all settings in Wolfenstein in game menu.

but the problem seems to relate to performance, something is strangling the CPU power somehow.

thanks in advance.