You might want to research LockWindowUpdate. It lets you lock the window to stop the form from painting, then you can use it after you're done changing the textbox to unlock the form and continue painting. I've used it in the TextChanged event, so I'm not sure how it works if you are using KeyUp, KeyDown, or KeyPress.
Here are some articles
]that use it.
Private Declare Function LockWindowUpdate Lib "user32" (ByVal hWnd As Integer) As Integer
Private Sub richtextbox_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) _
Hope this helps.