The material editor preview displays the material as a whole. You will actually paint and blend each layer separately.
You shouldn’t have to paint the entire landscape with each, just a small portion until the shaders start to compile and then wait for them to finish. Once you have painted with them all it should be working fine. Usually at this point it’s best practice to paint the entire landscape with one of the textures, which ever layer will cover most of the landscape. Then start painting and blending in the other textures.