If you obtain a model with thousands or even millions of faces it should be simplified before import into Unity because that is unnecessarily too large. Especially for AR applications on mobile devices, you do not want (or need) such complex models.

We cannot go into much detail here, but briefly, for example, using Blender you can simplify a model as follows:

1. If necessary, download and install Blender from https://www.blender.org/ website.
2. Open Blender and delete the default objects (press on keyboard A, then A again, then X to delete).
3. Import the model (File | Import | file type), locate your model and press Import.
4. If needed, scale the model so it's a manageable size on the screen (move mouse into the viewport, press S, and slide the mouse to scale, right-click when satisfied).
5. Ensure you're in Object Mode (see bottom toolbar), and in the Inspector on the right, find the wrench-icon to access the modifiers.
6. Click Add Modifier and choose Decimate (see the following screenshot).
7. The current number of faces in the mesh is shown in the Modifier pane. In the Ratio input field enter a number, such as 0.1 to reduce the faces to one-tenth. Press Apply.
8. Export the model as .fbx (File | Export | FBX).

In addition, there are several packages that let you simplify models directly in the Unity editor, such as Simple LOD (https://www.assetstore.unity3d.com/en/#!/content/25366). The caveat there is that the model still needs to be of a reasonable size to import, in the first place; otherwise, Unity will get bogged down. Simple LOD is intended to let you generate a series of simpler meshes at multiple levels of detail (LOD) for optimization and use, depending on the object distance from the in-scene camera.