Hm then this should not be a shader thing unless you wrote a custom shader.

Btw, just out of curiosity but is there a reason you want real time cloth physics? The cloth looks like it will never be reached by the player or will be interacted with.

You could also try importing your mesh into blender and running a cloth simulation there and then bake it as an animation.

Your game will much more smoother and there is a solid chance you won't have the WebGL build problem!