As Luos said, mainly the latter.
There is a tiny performance positive to certain uses.
Having say 3 materials that use an image for albedo and normal but everything else is the same. It is slightly better to have one be the master and instance to replace the images in the other 2 than to create them from scratch.
(still far more of an improvement in workflow)
Copy the level you’re working with and paint things with the alternative materials there.
Run each map and hit tilde (~) and type in
stat SceneRendering
to keep an eye on what might be tying up resources in the visuals department.