Add a component to set the target frame rate. Unity defaults to 30 fps

on mobile, but ARKit looks best at 60 Hz.

Assets/Scripts/SetTargetFramerate.cs

+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+/// <summary>
+/// Simple MonoBehaviour to set the Application's targetFrameRate
+/// </summary>
+public class SetTargetFramerate : MonoBehaviour
+{
+    [SerializeField]
+    [Tooltip("Sets the application's target frame rate.")]
+    int m_TargetFrameRate = 60;
+
+    /// <summary>
+    /// Get or set the application's target frame rate.
+    /// </summary>
+    public int targetFrameRate
+    {
+        get { return m_TargetFrameRate; }
+        set
+        {
+            m_TargetFrameRate = value;
+            SetFrameRate();
+        }
+    }
+
+    void SetFrameRate()
+    {
+        Application.targetFrameRate = targetFrameRate;
+    }
+
+    void Start()
+    {
+        SetFrameRate();
+    }
+}

Assets/Scripts/SetTargetFramerate.cs.meta

+fileFormatVersion: 2
+guid: 1eb9777efa8174381b01d319232f7df5
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: