処理の重いSQLを発行すると画面が固まってしまう

Imports System.Threading

Public Class Form1
    Inherits System.Windows.Forms.Form

    Delegate Function HeavyRoutineDelegate(ByVal sleepSecond As String) As Object

    Private delegateInstance As HeavyRoutineDelegate

    Private Sub Button1_Click( _
    ByVal sender As System.Object, ByVal e As System.EventArgs) _
    Handles Button1.Click
        delegateInstance = New HeavyRoutineDelegate(AddressOf Class1.HeavyRoutine)
        delegateInstance.BeginInvoke(Me.TextBox1.Text, _
            New AsyncCallback(AddressOf HeavyRoutineCallback), Nothing)
        Me.Enabled = False
    End Sub

    Private Sub HeavyRoutineCallback(ByVal ar As IAsyncResult)
        Dim obj As Object = delegateInstance.EndInvoke(ar)
        If TypeOf obj Is Exception Then
            Dim ex As Exception = CType(obj, Exception)
            MsgBox(ex.ToString(), MsgBoxStyle.Critical)
        End If
        Me.Enabled = True
    End Sub

End Class

Public Class Class1
    '例外テストのためにわざと引数をString型にしている
    Public Shared Function HeavyRoutine(ByVal sleepSecond As String) As Object
        Try
            System.Threading.Thread.Sleep(CInt(sleepSecond) * 1000)
            Return Nothing
        Catch ex As Exception
            '別スレッドなので例外は確実にキャッチ
            Return ex
        End Try
    End Function
End Class