how to load DataGridComboBoxColumn with MVVM

I implemented a datagrid using mvvm and here is how my viewmodel and view looks like as below. my problem is that when i load the products, they will be loaded fast but since each product has a category and manufacturer, it will bind all those in each combobox items. so it means that if i have 100 products, 1000 categories and 1000 manufacturers. it will load and bind those 100 times for each 1000 items. this is causing a big delay in loadtime and also datagrid looks heavy as i try to scroll up-down or left-right, there will be a significant slowness. How can i solve this problem? thanks for your advises.

Public Class ProductsVM

    Private _SearchKeyword As String
    Public Property SearchKeyword() As String
        Get
            Return _SearchKeyword
        End Get
        Set(value As String)
            If _SearchKeyword <> value Then
                _SearchKeyword = value

                RaisePropertyChanged()
            End If
        End Set
    End Property

     Private m_Products As ObservableCollection(Of Product)
    Public Property Products() As ObservableCollection(Of Product)
        Get
            Return m_Products
        End Get
        Set(value As ObservableCollection(Of Product))
            m_Products = value
        End Set
    End Property

    Private m_Categorys As ObservableCollection(Of Category)
    Public Property Categories() As ObservableCollection(Of Category)
        Get
            Return m_Categorys
        End Get
        Set(value As ObservableCollection(Of Category))
            m_Categorys = value
        End Set
    End Property

    Private m_Manufacturers As ObservableCollection(Of Manufacturer)
    Public Property Manufacturers() As ObservableCollection(Of Manufacturer)
        Get
            Return m_Manufacturers
        End Get
        Set(value As ObservableCollection(Of Manufacturer))
            m_Manufacturers = value
        End Set
    End Property

    Public Sub New()

    End Sub

    Private ButtonClick As relaycommand
    Public Property NewProperty() As relaycommand
        Get
            Return ButtonClick
        End Get
        Set(ByVal value As relaycommand)
            ButtonClick = value
        End Set
    End Property

    Protected Overrides Async Sub GetData()
        Try

            Dim _dboProducts As New ObservableCollection(Of Product)()

           Dim dboProducts__1 As New List(Of Product)
           dboProducts__1 = Await (From c In dboProducts Where c.Name.ToLower.Contains(SearchKeyword.Trim) Order By c.ProductId).ToListAsync()


            Products = dboProducts__1
            RaisePropertyChanged("Products")

            Dim _dboManufacturers As New ObservableCollection(Of Manufacturer)()
            Dim dboManufacturers__1 = Await (From c In mycontext.Manufacturer Order By c.ManufacturerId).ToListAsync()


            Manufacturers = dboManufacturers__1
            RaisePropertyChanged("Manufacturers")

            Dim _dboCategorys As New ObservableCollection(Of Category)()
            Dim dboCategorys__1 = Await (From c In mycontext.Category Order By c.CategoryId).ToListAsync()

            Categories = dboCategorys__1
            RaisePropertyChanged("Categories")

        Catch ex As Exception

        End Try

    End Sub

End Class

<DataGrid.RowStyle><Style TargetType="{x:Type DataGridRow}"><Setter Property="cst:DataGridRowBehavior.IsDataGridRowFocussedWhenSelected" Value="true"/></Style></DataGrid.RowStyle><DataGrid.Columns><DataGridTextColumn Header="ProductId"     Binding="{Binding  ProductId,Mode=OneWay}" IsReadOnly="True"  Width="100" ></DataGridTextColumn><DataGridComboBoxColumn Header="Manufacturer" Width="400"  filter:DataGridComboBoxExtensions.UserCanEnterText="True"
                            DisplayMemberPath="Name"
                            SelectedValueBinding="{Binding ManufacturerId}"
                            SelectedValuePath="ManufacturerId"><DataGridComboBoxColumn.ElementStyle><Style TargetType="ComboBox"><Setter Property="ItemsSource" Value="{Binding Path=DataContext.Manufacturers, RelativeSource={RelativeSource AncestorType=DataGrid}}" /></Style></DataGridComboBoxColumn.ElementStyle><DataGridComboBoxColumn.EditingElementStyle><Style TargetType="ComboBox"><Setter Property="ItemsSource" Value="{Binding Path=DataContext.Manufacturers, RelativeSource={RelativeSource AncestorType=DataGrid}}" /></Style></DataGridComboBoxColumn.EditingElementStyle></DataGridComboBoxColumn><DataGridComboBoxColumn Header="Category" Width="400"  filter:DataGridComboBoxExtensions.UserCanEnterText="True"
                            DisplayMemberPath="Name"
                            SelectedValueBinding="{Binding CategoryId}"
                            SelectedValuePath="CategoryId"><DataGridComboBoxColumn.ElementStyle><Style TargetType="ComboBox"><Setter Property="ItemsSource" Value="{Binding Path=DataContext.Categories, RelativeSource={RelativeSource AncestorType=DataGrid}}" /></Style></DataGridComboBoxColumn.ElementStyle><DataGridComboBoxColumn.EditingElementStyle><Style TargetType="ComboBox"><Setter Property="ItemsSource" Value="{Binding Path=DataContext.Categories, RelativeSource={RelativeSource AncestorType=DataGrid}}" /></Style></DataGridComboBoxColumn.EditingElementStyle></DataGridComboBoxColumn></DataGrid.Columns></cst:CustomDataGrid></Grid>

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