procedure Wait(Time: Word);
  var Start: Cardinal;
begin
  Start := GetTickCount;
  while GetTickCount-Start <= time do begin
    Application.ProcessMessages;
    Sleep(0);
  end;
end;

procedure TForm1.ShowText(Text: String);
  var i : Integer;
begin
  Label1.Caption := '';
  for i := 1 to Length(Text) do begin
    Label1.Caption := Label1.Caption + Text[i];
    Wait(500);
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowText('Hallo');
end;