Low-hanging fruit of query optimization, these indices were missing

db/migrate/20170406215816_add_notifications_and_favourites_indices.rb

@@ -0,0 +1,7 @@
+class AddNotificationsAndFavouritesIndices < ActiveRecord::Migration[5.0]
+  def change
+    add_index :notifications, [:activity_id, :activity_type]
+    add_index :accounts, :url
+    add_index :favourites, :status_id
+  end
+end