Microsoft states that DirectX 11 is downward compatible to DirectX9, but, in my experience, it is not. One library that nwn uses seems not to be present when only installing DX11. So you have to install both alongside each other which is perfectly possible.
OTOH, lag issues are usually not caused by DX. If it is the DX culprit, NWN simply won't start with a "missing dll" error. Lag problems are quite more probable due to graphics driver issues.
HTH
Greetings
Skildron