unfortunatelly i do not know much about xorg.conf file. or why it would cause system to slow down. it's been too long since i used one. i am sure someone else will know more. though these files are not used often lately. but they are still used on certain occasions.And can you please comment on what I said about creating the xorg.conf file in the X11 folder?
As I said, all runs fine (including Unity) until I create it.
Even without modifing anything.
edit: try askign on ubuntu IRC channel or maybe xorg have their own IRC channel.