Why is this VBA code (that I wrote) executing so slow?

I've created a user form for people to enter data into. All is working well in the workbook, except this form is slowing things down. It takes WAY too long to process the data that is entered (and thus allocated).
Any suggestions on how to speed this up?

Private Sub cboDept1_Change()
Dim idx As Long
Dim I As Long

idx = cboDept1.ListIndex

If idx <> -1 Then
With Sheet3
For I = 3 To 22
Me.Controls("Label" & I + 12).Caption = .Range("C" & I).Offset(0, idx).Text
Next I
End With
End If

End Sub

Private Sub cmdCancel_Click()
Unload Me
End Sub

Private Sub cmdClearForm_Click()
Call UserForm_Initialize
End Sub

Private Sub cmdOK4_Click()
Application.ScreenUpdating = False
Dim rng As Range, wd As Worksheet, r As Long
Set wd = ActiveWorkbook.Sheets("IndicatorDB")
r = wd.Range("A" & Rows.Count).End(xlUp).Row
For I = 1 To 20
If Me.Controls("TextBox" & I).Value <> "" Then

r = r + 1: Set rng = wd.Range("A" & r)
rng.Offset(-1, 0).EntireRow.Copy: rng.PasteSpecial xlPasteFormulas

rng.Value = cboName1.Value
rng.Offset(0, 1) = cboDept1.Value
rng.Offset(0, 2).Value = Me.Controls("Label" & I + 14).Caption
rng.Offset(0, 3).Value = DTPicker4.Value
rng.Offset(0, 4).Value = Me.Controls("TextBox" & I).Value
Set rng = rng.Offset(1)
End If
Next I


Sheets("Forms").Select
Unload Me
UserForm2.Show
Application.ScreenUpdating = True
End Sub

Private Sub UserForm_Initialize()
With Worksheets("ADMIN")
cboDept1.List = .Range("F4", .Range("F" & Rows.Count).End(xlUp)).Value
End With

With Worksheets("Employees")
cboName1.List = .Range("A1", .Range("A" & Rows.Count).End(xlUp)).Value
End With

End Sub

Sub SlowWithFlicker()

Dim rCell As Range

For Each rCell In Range("A1:A15000")

rCell.Select

Next rCell

End Sub



Sub FastWithNoFlicker()

Dim rCell As Range

Application.ScreenUpdating = False

For Each rCell In Range("A1:A15000")

rCell.Select

Next rCell

Application.ScreenUpdating = True

End Sub