Application.screenupdating causing program lagg

Hi,
I made a program in Private Sub Worksheet_SelectionChange(ByVal Target As Range) event, so it will run some code everytime i select a cell.

The problem is, I get a lagg (ie, the excel sheet "flashes") everytime i click on a cell.
I am 100% sure the problem is caused by application.screenupdating, since if I remove that line, I don;t lagg; I get instant response

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

    Application.ScreenUpdating = False
    'some code here
    'checked with Debug.Print Application.ScreenUpdating, which shows FALSE, just as it's supposed to


    'some more code here
    'checked again with Debug.Print Application.ScreenUpdating, which shows FALSE, just as it's supposed to

    'checked with Debug.Print Application.ScreenUpdating, which shows FALSE, just as it's supposed to
    Application.ScreenUpdating = True
    'checked with Debug.Print Application.ScreenUpdating, which shows TRUE, just as it's supposed to
End Sub

Any help would be appciated
Thanks