A good place to start is using the -dxlevel 81 launch option. To access the launch options go to your applications list in steam, right click on the application, choose properties, and then choose set launch options. Type -dxlevel 81 into the launch options, and then open the game. After you have opened the game once with this launch option remove it from the launch options, as it will automatically apply from now on, and if you do not remove it, it will reset your other graphics options each time.
Just to make sure that the autoexec is working look in your graphics settings and make sure that everything is on the lowest settings, HDR should definitely be turned off if you want to raise your frame rate. If it doesn't hurt your framerate you should always turn on wait for vertical sync, as this lines up the frames you do render with the blanking interval of your LCD, making sure you get the most out of the frames your computer renders.