This guide might be helpful
http://www.tweakguides.com/GRAW_3.html
Mouse/Keyboard Lag: Typically the most common cause of mouse or keyboard lag is low framerates, so check your framerate using FRAPS and if it dips below 15 or so then you will start getting noticeable control lag - lower various settings such as Resolution and/or Texture Quality and/or Post Effects for example to improve framerate. However if this doesn't work, check the Neat Stuff & Conclusion section of the guide and impement the 'Max Frames to Render Ahead' tweak. Furthermore, apparently some people have found this fix for keyboard lag to work for them. Finally, check the Hardware Cursor tweak under the Advanced Tweaking section.