Public Interface IPerson
    Sub IPersonMethod()
End Interface

Public Class Person
    Public Sub PersonMethod()
    End Sub
    Public Sub SPersonMethod()
    End Sub
    Public Overridable Sub OPersonMethod()
    End Sub
End Class

Public Class Manager
    Inherits Person
    Implements IPerson

    Public Shadows Sub PersonMethod()
    End Sub

    Public Overrides Sub OPersonMethod()
    End Sub

    Public Sub IPersonMethod() Implements _
        IPerson.IPersonMethod
    End Sub
End Class

Private Sub btnOverrides_Click(ByVal sender As _
    System.Object, ByVal e As System.EventArgs) Handles _
    btnOverrides.Click
    Dim start_time As DateTime
    Dim stop_time As DateTime
    Dim elapsed_time As TimeSpan
    Dim num_trials As Long = Long.Parse(txtNumTrials.Text)
    Dim obj As New Manager
    lblOverrides.Text = ""
    Me.Cursor = Cursors.WaitCursor
    Application.DoEvents()

    start_time = Now
    For i As Long = 1 To num_trials
        obj.OPersonMethod()
    Next i
    stop_time = Now
    elapsed_time = stop_time.Subtract(start_time)
    lblOverrides.Text = _
        elapsed_time.TotalSeconds.ToString("0.0000")
    Me.Cursor = Cursors.Default
End Sub

Method	Time	Compiled Code
Person.PersonMethod	2.2332	000000f7 mov ecx,dword ptr [ebp-1Ch] 000000fa cmp dword ptr [ecx],ecx 000000fc call dword ptr ds:[007F7170h] 00000102 nop
Manager.PersonMethod	2.2332	000000f7 mov ecx,dword ptr [ebp-1Ch] 000000fa cmp dword ptr [ecx],ecx 000000fc call dword ptr ds:[007F72B4h] 00000102 nop
Manager As Person (Dim obj As Person = New Manager)	2.0630	000000f7 mov ecx,dword ptr [ebp-1Ch] 000000fa cmp dword ptr [ecx],ecx 000000fc call F868400B 00000101 nop
Object (DirectCast(obj, Manager).PersonMethod)	2.9242	000000f7 mov edx,dword ptr [ebp-1Ch] 000000fa mov ecx,7F726Ch 000000ff call 7107D95C 00000104 mov ecx,eax 00000106 cmp dword ptr [ecx],ecx 00000108 call F8683F5B 0000010d nop
IPerson.PersonMethod	2.4936	000000f7 mov ecx,dword ptr [ebp-1Ch] 000000fa mov eax,dword ptr [ecx] 000000fc mov eax,dword ptr [eax+0Ch] 000000ff mov eax,dword ptr [eax+000000F4h] 00000105 call dword ptr [eax] 00000107 nop
Shadows (Manager.SPersonMethod)	2.2032	000000fc mov ecx,dword ptr [ebp-1Ch] 000000ff cmp dword ptr [ecx],ecx 00000101 call dword ptr ds:[007F7174h] 00000107 nop
Overrides (Manager.OPersonMethod)	2.1831	000000f7 mov ecx,dword ptr [ebp-1Ch] 000000fa mov eax,dword ptr [ecx] 000000fc call dword ptr [eax+38h] 000000ff nop

It's good news that the parent (Person) and derived class (Manager) methods take the same amount of time. Their decompiled code is almost identical so that makes sense.

It's interesting that using a Manager object declared as a Person is faster. The decompile code is slightly simpler. I suspect some compile-time calculations have made the simplification. (Someone who understands this stuff, let me know.)

Casting from a generic Object to a Manager takes longer. That makes sense because there's a whole extra step there.

It also makes some sense that using the IPerson interface adds some overhead, although it seems like the compiler might have been able to avoid that.

It's odd that the Shadows and Overrides methods are faster than the original methods, particularly since the decompiled code looks very similar to the Person and Manager versions. The differences are small but consistent.