Can I speed up a for each loop

Question

This code seems to be very simple, but it's taking a long time to run. I've made a sample project with only the basic code included and it's still very slow.

I have a form with a button and a panel. At runtime the panel is dynamically filled with approx 1700 custom User controls. The custom user control contains one panel and one property. Clicking the button on the form causes each user control to swap its color. This action should happen all at once, but it's so slow that I can watch each user control switch color, row, by row, one at a time

Can I suspend screen updating until after the whole for each loop runs, rather than updating each panel one by one?

User Control code...

   Public Class ucBox
    Private _TurnedOn As Boolean
    Public Property TurnedOn() As Boolean
        Get
            Return _TurnedOn
        End Get
        Set(ByVal value As Boolean)
            _TurnedOn = value
            If TurnedOn Then
                PnlBox.BackColor = Color.Red
            Else
                PnlBox.BackColor = Color.LightGray
            End If
        End Set
    End Property

    Private Sub PnlBox_Click(sender As Object, e As EventArgs) Handles PnlBox.Click
        If TurnedOn Then
            TurnedOn = False
        Else
            TurnedOn = True
        End If
    End Sub
End Class

Form Code...

    Public Class Form1
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles Me.Load
        Dim Rw As Int16 = 0, Col As Int16 = 0, x As Int16
        Dim Box As ucBox
        With btnTest
            .Text = "Test Me"
        End With

        While Rw < PnlMain.Height - 10
            While Col < PnlMain.Width - 10
                Box = New ucBox
                x = x + 1
                With Box
                    .Top = Rw
                    .Left = Col
                    .TurnedOn = False
                    .Name = "Box_" & x
                    Col += .Width + 4

                End With

                PnlMain.Controls.Add(Box)
                Box = Nothing
            End While
            Col = 0
            Rw += 24
            If x > 2000 Then Exit While
        End While
        Debug.Print("There are " & x & " boxes.")
    End Sub

    Private Sub btnTest_Click(sender As Object, e As EventArgs) Handles btnTest.Click
        Dim StartTime As Date = Now

        For Each b As ucBox In PnlMain.Controls
            If b.TurnedOn Then
                b.TurnedOn = False
            Else
                b.TurnedOn = True
            End If
        Next

        Debug.Print("Test took " & Now.Subtract(StartTime).TotalMilliseconds)
    End Sub
End Class

I appreciate any ideas.