Option Explicit

Public Sub CheckAndConcatenate()

Dim t
Dim sh As Worksheet
Dim rngCheck As Range
Dim strColumn1 As String
Dim strColumn2 As String
Dim strConcat As String
Dim cell As Range

t = Timer

Application.Calculation = xlCalculationManual
Application.ScreenUpdating = False
Application.EnableEvents = False

Set sh = ActiveSheet
strColumn1 = "A"
strColumn2 = "B"
strConcat = "PR"
Set rngCheck = sh.Range(strColumn1 & "1:" & strColumn1 & LastCellOfColumn(sh, strColumn1))

For Each cell In rngCheck
    If LenB(cell.Value2) > 0 Then
        sh.Range(strColumn2 & cell.Row).Formula = "=""" & strConcat & """&" & cell.Address
        'sh.Range(strColumn2 & cell.Row).Value2 = strConcat & cell.Value2 'However, I think this is the way better rather than to use formula.
    End If
Next

Application.Calculation = xlCalculationAutomatic
Application.ScreenUpdating = True
Application.EnableEvents = True

Debug.Print Timer - t

End Sub

Private Function LastCellOfColumn(Sheet As Worksheet, Column As String) As Long

Dim rng As Range

Set rng = Sheet.Range(Column & Sheet.Rows.Count)
If LenB(rng.Value2) > 0 Then
    LastCellOfColumn = rng.Row
Else
    LastCellOfColumn = rng.End(xlUp).Row
End If

End Function