Realm在更新多个对象时变慢

In my app the user can select multiple contacts in a collectionview. when he selects the property "isSelected" will me set to true and the collectionview refreshes the selected cell. Here I can recognize a small delay between selection and the highlighting of the cell. But in the next step I create a group with the selected contacts and in the end I set the property "isSelected" to false. This takes an non acceptable amount of time for 50 objects (5 seconds) and needs to be tuned.In my app the user can select multiple contacts