Function СчитаемСумму_1(MyText As String) As Long
    Const ss As String = "абвгдеёжзийклмнопрстуфхцчшщъыьэюя"
    Dim j As Long, i As Long
    
    For j =  1  To Len(MyText)
        i = i + InStr( 1 , ss, Mid$(MyText, j,  1 ))
    Next
    СчитаемСумму_1 = i
End Function

Function СчитаемСумму_2(MyText As String) As Long
    Dim j As Long, i As Long, k As Integer
    
    For j =  1  To Len(MyText)
        k = Asc(Mid$(MyText, j,  1 ))
        If k <  224  Then
            If k =  184  Then i = i +  7 
        ElseIf k <  230  Then
            i = i + k -  223 
        Else
            i = i + k -  222 
        End If
    Next
    СчитаемСумму_2 = i
End Function

Function СчитаемСумму_3(MyText As String) As Long
    Dim j As Long, i As Long, k As Integer, m() As Byte
    
    m = MyText
    For j =  0  To UBound(m) -  1  Step  2 
        k = m(j)
        If k >  47  Then
            If k <  54  Then
                i = i + k -  47 
            ElseIf k <  80  Then
                i = i + k -  46 
            ElseIf k =  81  Then
                i = i +  7 
            End If
        End If
    Next
    СчитаемСумму_3 = i
End Function

    Dim t As Double, ss As String, s As String, i As Long, sss As String

    ss = ... ' Заполняем переменную 3300033 символами

    ss = LCase$(ss)

    t = Timer
    i = СчитаемСумму_1(ss)
    t = Timer - t
    sss = "1: Символов = " & Len(ss) & ",  Сумма = " & i & ",  Время, с = " & Format$(t, "#,##0.000000") & vbCrLf

    t = Timer
    i = СчитаемСумму_2(ss)
    t = Timer - t
    sss = sss & "2: Символов = " & Len(ss) & ",  Сумма = " & i & ",  Время, с = " & Format$(t, "#,##0.000000") & vbCrLf

    t = Timer
    i = СчитаемСумму_3(ss)
    t = Timer - t
    sss = sss & "3: Символов = " & Len(ss) & ",  Сумма = " & i & ",  Время, с = " & Format$(t, "#,##0.000000")
    
    Debug.Print sss
    MsgBox sss

Function СчитаемСумму_3(MyText As String) As Long
    Dim j As Long, i As Long, k As Integer, m() As Byte
    
    m = MyText
    For j =  0  To UBound(m) -  1  Step  2 
        If m(j +  1 ) =  4  Then
            k = m(j)
            If k >  47  Then
                If k <  54  Then
                    i = i + k -  47 
                ElseIf k <  80  Then
                    i = i + k -  46 
                ElseIf k =  81  Then
                    i = i +  7 
                End If
            End If
        End If
    Next
    СчитаемСумму_3 = i
End Function