private void AddFilesToTheTree(TreeViewItem item, List<BackupFile> _backupFileList)
        {
            foreach (BackupFile addition in _backupFileList)
            {                
                var subitem = new TreeViewItem
                {
                    DataContext = addition,
                    Header = addition.Name
                };
                
                item.Items.Add(subitem);
            }
        }

 Вызывающим потоком должен быть STA, поскольку этого требуют большинство компонентов UI

using (MyTreeView.Items.DeferRefresh())
    item.Items.Add(subitem);

 private void AddFilesToTheTree(TreeViewItem item, List<BackupFile> _backupFileList)
        {
            List<TreeViewItem> treeViewItemList = new List<TreeViewItem>();
            foreach (BackupFile addition in _backupFileList)
            {
                var subitem = new TreeViewItem
                {
                    DataContext = addition,
                    Header = addition.Name
                };

                treeViewItemList.Add(subitem);
            }
            
            using (FolderSelectionTree.Items.DeferRefresh())
            {
                foreach (TreeViewItem subitem in treeViewItemList)
                    item.Items.Add(subitem);
            }
        }

item.Items.Add(subitem)

using (item.Items.DeferRefresh())
            {
                foreach (TreeViewItem subitem in treeViewItemList)
                    item.Items.Add(subitem);
            }

Cannot change or check the contents or Current position of CollectionView while Refresh is being deferred.

TreeView tv = new TreeView();
ItemCollection collection = tv.Items;

foreach (BackupFile addition in _backupFileList)
            {                
                var subitem = new TreeViewItem
                {
                    DataContext = addition,
                    Header = addition.Name
                };
                
                collection.Add(subitem);
            }


item.ItemsSource = collection;

MyTreeView.Items.Clear();
collection = MyTreeView.Items;

TreeView tv = new TreeView();
tv.Resources = MyTreeView.Resources;
ItemCollection collection = tv.Items;

foreach (BackupFile addition in _backupFileList)
            {                
                var subitem = new TreeViewItem
                {
                    DataContext = addition,
                    Header = addition.Name
                };
                
                collection.Add(subitem);
            }


item.ItemsSource = collection; <----- [color=red]ВОТ ЗДЕСЬ ЗАДЕРЖКА НА 15 СЕКУНД, т.к. в коллекции 100 элементов[/color]