To make the world of The Witcher more realistic, we've created a real mod that removes all of the annoying collisions with trees.  If you're using the No Trees No Collisions mod, you will be asked to remove a certain file from your game directory. That is all you need to do. And it will give you a big fps boost. 

To install, locate your game directory. Then, delete the required files. Prior to deletion, you can choose whether to remove all foliage, only the grass, or the trees. Afterward, you can now install this mod which will remove the foliage collisions aside from Roach.