Delphi DX 10.3 VS Delphi 6 performances - delphi

Question

I have been using Delphi 6 for a very long time, and I recently started using version Delphi DX Rio 10.3.
Against all my expectations, it seems to me that loading items on standard controls (ComboBox, Listbox and so on) in DX RIO is significantly slower (at runtime) than in D6 (both in DEBUG and RELEASE mode, after activating all optimizations).
I tried using BeginUpdate / EndUpdate, without changing the speed difference between the environments.
I would like to ask if anyone has any suggestions for improving DX 10.3 Combobox/Listbox performances. The specific problem I'm trying to solve in my real application is to load some dozens of items in 10-20 combos/listboxes controls on my form.
I wrote a very simple test for a standard COMBO. The same program works in both D6 and DX RIO.
On my development machine (Win 10 pro) the program compiled with D6 takes about 1 second, while the program compiled with DX RIO takes about four times as much.
Many thanks in advance for your answers.
{$A+,B-,C+,D+,E-,F-,G+,H+,I+,J-,K-,L+,M-,N+,O+,P+,Q-,R-,S-,T-,U-,V+,W-,X+,Y+,Z1}
unit Unit1;
interface
uses Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs, StdCtrls;
type
TForm1 = class(TForm)
cb: TComboBox;
txt_count: TLabel;
btn: TButton;
procedure btnClick(Sender: TObject);
end;
var Form1: TForm1;
implementation
{$R *.dfm}
var dt_start_timer : TDatetime;
procedure start_timer;
begin
dt_start_timer := now
end;
function delta_timer_msec : longint;
begin
result := round((now - dt_start_timer) * 24*60*60*1000)
end;
procedure execute(father : TForm;cb : TComboBox;txt : TLabel;bo_disable_update : boolean);
const
MAX = 100;
ITEMS_COUNT = 1000;
var i, j : integer;
begin
start_timer;
if bo_disable_update then cb.Items.BeginUpdate;
for i := 1 to MAX do begin
cb.Items.Clear;
for j := 1 to ITEMS_COUNT do cb.Items.Add('text-' + intToStr(j))
end;
if bo_disable_update then cb.Items.EndUpdate;
txt.Caption := intToStr(delta_timer_msec) + ' msecs'
end;
procedure TForm1.btnClick(Sender: TObject);
begin
execute(self, cb, txt_count, {disable_update}TRUE)
end;
end.
object Form1: TForm1
Left = -6
Top = 117
Width = 449
Height = 350
Caption = 'Form1'
Color = clBtnFace
Font.Charset = DEFAULT_CHARSET
Font.Color = clWindowText
Font.Height = -11
Font.Name = 'MS Sans Serif'
Font.Style = []
OldCreateOrder = False
PixelsPerInch = 96
TextHeight = 13
object txt_count: TLabel
Left = 198
Top = 32
Width = 44
Height = 13
Caption = 'txt_count'
end
object cb: TComboBox
Left = 42
Top = 28
Width = 145
Height = 21
ItemHeight = 13
TabOrder = 0
Text = 'cb'
end
object btn: TButton
Left = 24
Top = 70
Width = 251
Height = 139
Caption = 'btn'
TabOrder = 1
OnClick = btnClick
end
end