The IGP has very fast access to RAM, faster than the discrete video card. That is why it is faster in parts.
You might be able to get full speed with this build:
http://www.mediafire.com/download/kxs2bu...01-x64.zip
You might be able to get full speed with this build:
http://www.mediafire.com/download/kxs2bu...01-x64.zip