As strange as this may sound, running a game on low graphics can actually slow your framerate. It has happened at times. You need to update your graphics and cpu drivers quickly and use DX10/11 if possible...
I have figured out the problem(at least for me). When the game is in DX9 it seems to have major performance problems. I left the game and changed the start up settings in steam to DX11 and the game stopped having performance issues. I have not bothered to change the settings in-game to see if the game has a problem changing them in-game as I am too scared that the problem will return.
I can now run the game on max settings with 2560x1440 downsampled to 1080p with sweet fx at 80+fps.
1.Leave game if you are in it
2.In steam select "Setup Deus Ex:Human Revolution"
3.Select Use Direct X 11
I might have done some other things but this is what I remember doing.
I checked your thread and you are correct. ENB mod drops the game to 5fps for me as well.