(Cross-posting from https://github.com/godotengine/godot/issues/39014.)
Real-time lights will always be expensive on mobile devices, especially when shadows are enabled. If you want shadows on mobile, it's best to use baked lightmaps. The current lightmapper isn't great but there will be a new one in 3.2.4: https://github.com/godotengine/godot/pull/44628
Also, consider rendering the viewport at a lower resolution using the
viewport stretch mode and setting a relatively low base resolution. This improves performance tremendeously and is often needed to achieve good 3D performance on mobile. See Multiple resolutions in the documentation for more information.
The optimization tutorials are worth reading if you want to know more optimization tips.