If you're having performance issues with The Witcher 3, then there's a good chance that you don't have the recommended hardware required to play the game on full ultra settings. That said, you have options to make the game run smoothly by sacrificing a few graphical settings.
Anyone running a 600 or 700 series NVidia card will find this guide helpful. If you're running a 900 series, there's very little chance that you'll be having issues with the game's performance at its recommended settings.
So here's what you can do to get the most out of the game without turning it all down to MEDIUM or LOW:
In the 'Graphics' Menu, bump everything to HIGH except for Foliage Visibility Range, which you should keep on MEDIUM.
Make sure Nvidia HairWorks is disabled.
Make sure the Display Mode is set to Full Screen–this should net you a good performance gain.
You'll also want to set the framerate cap under Maximum Frames per Second to 30fps. This should give you a fluid display without any stuttering to ruin your experience. Unless you're running an Nvidia 970 or above, you will not be able to get 60fps at these settings.
In 'Postprocessing', disable everything except Motion Blur, Sharpness, HBAO+ and Lights.
If you're feeling particularly adventurous with your framerate you can cap it to 40 by going into the Witcher 3 folder in My Documents and locating the file called user.settings. Open it with a notepad and hit Ctrl-F to search for LimitFPS. Set that to 40. Here's the exact directory:
%USERPROFILE%\Documents\The Witcher 3\user.settings