Unity efficiently convert RenderTexture to Texture3D.cs

//input RenderTexture 'rt3D'

Texture3D output = new Texture3D(width, height, depth, rt3D.graphicsFormat, TextureCreationFlags.None);
//assuming rt3D format to be 8 bits
var a = new NativeArray<byte>(width * height * depth, Allocator.Persistent, NativeArrayOptions.UninitializedMemory);
AsyncGPUReadback.RequestIntoNativeArray(ref a, rt3D, 0, (_) =>
{
    output.SetPixelData(a, 0);
    output.Apply(updateMipmaps: false, makeNoLongerReadable: true);
    AssetDatabase.CreateAsset(output, "Assets/3dtex.asset");
    a.Dispose();
});