Deferred rendering path stopped to function after adding a menu

Hi, My scene (in Unity 2017.4.25 Windows) operated previously very well in deferred rendering path, usually the scene shows 60 fps with 1.5M tris. Then I add a simple log in menu, which loads the main scene. But after loading, the main scene acts like in forward rendering path because the scene shows now 40 fps and 4.5M tris. The graphic settings and camera settings in both scenes are set to deferred rendering path. The menu scene uses simple canvas to show log in, and it loads the main scene with SceneManager.LoadSceneAsync("MyScene", LoadSceneMode.Additive).