Load ComboBoxColumn with DataGrid load in WPF

Question

I've got a DataGrid containing a TemplateColumn containing a combo box, which looks like:

  <DataGridTemplateColumn Header="Initials" Width="50" >
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <ComboBox SelectedValuePath="Initials"  DisplayMemberPath="Initials" ItemsSource="{Binding Path=DataContext.Drivers, RelativeSource={RelativeSource AncestorType={x:Type Window}}}" >

                        </ComboBox>
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
            </DataGridTemplateColumn>

This works, but it seems like the ItemSource is only loaded the first time a user tries to edit the combobox selection, instead of when the DataGrid loads. This means the client hangs for a few when a combobox is selected. I want this to happen when the rest of the DataGrid is loaded. Is there a way to do this?

Edit: Just tried enabling UI Virtualization on the box per this question, and it didn't help. I made the following test case that has the same issues. In the xaml:

<DataGrid Name="TestDataGrid">
        <DataGrid.Columns>
        <DataGridTemplateColumn Header="Initials" Width="50">
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>

                    <ComboBox VirtualizingStackPanel.IsVirtualizing="True" ScrollViewer.IsDeferredScrollingEnabled="True" ScrollViewer.CanContentScroll="True" SelectedValuePath="Initials" SelectedValue="{Binding Path=Initials, Mode=TwoWay, IsAsync=True}" DisplayMemberPath="Initials" ItemsSource="{Binding Path=DataContext.Drivers, RelativeSource={RelativeSource AncestorType={x:Type Window}}}" >
                        <ComboBox.ItemsPanel>
                            <ItemsPanelTemplate>
                                <VirtualizingStackPanel />
                            </ItemsPanelTemplate>
                        </ComboBox.ItemsPanel>
                    </ComboBox>

                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
        </DataGridTemplateColumn>
        </DataGrid.Columns>
    </DataGrid>

In the window's code behind:

 public TestWindow()
    {
        InitializeComponent();
        var _context = new TestContext();
        DataContext = _context;
        TestDataGrid.ItemsSource = _context.Atms;
    }

And in the data context:

   public ObservableCollection<Driver> Drivers { get => drivers; set => drivers = value; }

    public ObservableCollection<Driver> drivers;
    private ListCollectionView atms;

    public ListCollectionView Atms { get => atms; set => atms = value; }
    public TestContext()
    {
       drivers = Driver.GetDriversToday();
        var atms = AtmForecast.GetAtms(UpdateTotalCallback);
        ListCollectionView collection = new ListCollectionView(atms);

        Atms = collection;
    }
    public void UpdateTotalCallback(AtmForecast sender)
    {
        

    }

The driver list is super small too, ~5 entries, and it still freezes for a few seconds when opening the combo box at first. The one thing I have noticed is, in the debug output:

    mincore\com\oleaut32\dispatch\ups.cpp(2126)\OLEAUT32.dll!00007FFE8D6A1924: (caller: 00007FFE8D6A1A5A) ReturnHr(1) tid(8648) 8002801D Library not registered.
'RoutesApp.exe' (Win32): Loaded 'C:\Windows\System32\oleacc.dll'. Symbols loaded.
'RoutesApp.exe' (Win32): Loaded 'C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App\3.1.20\Accessibility.dll'. 
'RoutesApp.exe' (CoreCLR: clrhost): Loaded 'C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App\3.1.20\Accessibility.dll'. Symbols loaded.

This is the only output when a ComboBox is first clicked. Unsure if this dll can be manually loaded with the Window? I'm not sure how to do that if so.