DX11 makes the game run faster. It reduces the CPU stress and addresses more RAM than the DX9 32 bit client. Before you run the DX11 client be sure to update to the latest Nvidia/AMD drivers. Be sure to uninstall the old drivers first reboot then install the new drivers and reboot once more. If you keep having serious problems this means you either have really weak hardware or other drivers in your system needs also update (for laptops). And again be sure you have enough RAM and an appropriate DX11 capable GPU to run the DX11 client. Alternatively you can use the official Heavensward benchmark and you can switch between DX9/DX11 to see the differences. By default the DX11 client have some extra graphic options enabled that DX9 hasn't so this may be the reason that for some it runs slower.