漏れもテストした。

Private Sub Command1_Click()
  Dim sngTimer As Single
  Dim sTest As String
  Dim s As String
  Dim n As Long
  Dim nLoopMax As Long

  nLoopMax = 10000000
  m_sTest = String(nLoopMax, "*")

  sngTimer = Timer()
  For n = 1 To nLoopMax
    s = Left(sTest, n)
    's = Left$(sTest, n)
  Next

  Debug.Print "Timer=" & (Timer() - sngTimer)
End Sub

結果
Left$のとき Timer=3.625
Leftのとき Timer=6.438477