If you remove the comma from 40,000 to Warhammer 40000 Inquisitor - Martyr in the game's folder name.  It allowed me to add Warhammer_x64_11.exe in my Nvidia Control Panel and play the game with my Dedicated GPU.   It screws with the dir. path name but it works.  More importantly I think it's the key to help Devs provide a proper fix.  The older WH40K uses the comma without any issues.

I have the same issue. The supplied workaround by Cerberus doesn't work for me. Still getting sub 20 FPS on an i7, 16 GB, GTX765M

Edit: medium settings

Im using an MSI with a 980M and have no problems at highest settings.

The listing for Nvidia is a known issue but it can be easily worked around.

You can either set up in the Nvidia control panel do default to Nvidia processor for all applications: under 3d settings, manage 3d settings, select Global settings tab and select the Nvidia processor it will then set application default use to the Nvidia processor.

Or to set it manually right click the game app in steam select properties, local files tab, view local files right click the launch executable (Warhammer_x64_11) select run with graphics processor from the menu and select Nvidia high performance processor. (add a shortcut to this executable to your desktop and launch game from the shortcut)

Should sort out your issue.