Instructions on Appending a node group:-

On the top left, Click on File-->Append-->Navigate to the folder where you have downloaded the files -->Click on any of the .blend files-->NodeTree--> Select the Node group 

Alternatively, Simply copy the object from the downloaded .blend file, and paste it into your Scene. Blender also copies all the active materials being used on the object.

Procedural Materials could undoubtedly take a toll on your system, some basic optimization techniques could be: 

-> Disconnecting the normal, and roughness channels.

-> Change displacement from Displacement and Bump --> Displacement Only

-> Reducing the number of subdivisions on your mesh. 

-> Baking the textures

After making these changes, you may notice a significant improvement in viewport performance without much loss in quality. Remember that all of these parameters can be altered back for a final render. Hope you have fun with the materials