Is it possible to change the texture res within blender? I have 4K textures which I want to render in 2K for a preview rendering. Do I have to reload the textures in the material every time? my scene is big and heavy so I can't make 2 different ones for all materials without spending hours on it.
Render Properties > Simplify feature:
Moreover since 2.80 you can resize image directly in Image or UV Editor:
Note that you'll need to save (Alt+S) the image afterwards.