IIf is also harder to read. Most of the cost of a large project is in debugging and maintenance so anything that causes extra confusion is bad.

Private Sub cmdGo_Click()
Dim num_trials As Long
Dim i As Long
Dim start_time As Single
Dim stop_time As Single
Dim x As Integer

    num_trials = CLng(txtNumTrials.Text)
    lblIfThen.Caption = ""
    lblIIf.Caption = ""
    Screen.MousePointer = vbHourglass
    DoEvents

    start_time = Timer
    For i = 1 To num_trials
        If i = 0 Then
            x = 1
        Else
            x = 2
        End If
    Next i
    stop_time = Timer
    lblIfThen.Caption = Format$(stop_time - start_time, _
        "0.0000") & " seconds"
    DoEvents

    start_time = Timer
    For i = 1 To num_trials
        x = IIf(i = 0, 1, 2)
    Next i
    stop_time = Timer
    lblIIF.Caption = Format$(stop_time - start_time, _
        "0.0000") & " seconds"

    Screen.MousePointer = vbDefault
End Sub