[...]
 private
    { private declarations }
    procedure CountDown(Sender: TObject);
  public
    { public declarations }
  end;
[...]
{ TForm1 }
var
  GThreadCount: Integer = 0;
  Gfreq: Int64;
  GstartTime: Int64;
  GendTime: Int64;

procedure TForm1.CountDown(Sender: TObject); //Aktion am Ende eines Threads
begin
  Dec(GThreadCount);
  if ThreadCount = 0 then
  begin
    QueryPerformanceCounter(GendTime);
    ShowMessage('Die Routine benötigte etwa ' + IntToStr((GendTime - GstartTime) * 1000 div Gfreq) + 'ms');
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  DateienLst: TSearchRec;
  Thread1: TMyThread1;
begin
  QueryPerformanceFrequency(Gfreq);
  QueryPerformanceCounter(GstartTime);

  GThreadCount:= 0;
  if FindFirst(Directory + '*.txt', faAnyFile and not faDirectory, DateienLst) = 0 then
    try
      repeat
        Inc(GThreadCount);
        Thread1:= TMyThread1.Create(True);
        Thread1.FreeOnTerminate := True;
        Thread1.FDateienname:= Directory+DateienLst.Name;
        Thread1.OnTerminate:= @CountDown;// Prozedur die beim beenden des Threads ausgeführt wird
        Thread1.Resume;
      until FindNext(DateienLst) <> 0;
    finally
      SysUtils.FindClose(DateienLst);
    end;
end;