Repaint small part of form only using Delphi?

In my program I have a custom TPanel that slides out from above and has buttons and images on it. It slides out fine but when I go to hide it, it is very slow. I have tried several methods and all are the same result. If you notice below I tried Redraw thinking I could just redraw the trailing rect but this call redraws the entire form as does all the other methods I have tried. I even tried using scroll. Am I calling it wrong or is there a better way to just update the form part that needs updating as the panel moves up?

while z>mypanels[panel].Fstarttop do
begin
   if z-x<mypanels[panel].Fstarttop then z:=mypanels[panel].Fstarttop else z:=z-x;
   updaterect:=Rect(mypanels[panel].left,z+mypanels[panel].height,mypanels[panel].left+mypanels[panel].width,mypanels[panel].top+mypanels[panel].height);
   mypanels[panel].top:=z;
   //mypanels[panel].Repaint;
   //Application.ProcessMessages;  
   //mypanels[panel].ScrollBy(0,-x);
   //InvalidateRect(form1.Handle,@updaterect,FALSE);
   //Application.ProcessMessages;
   //RedrawWindow(form1.Handle,@updaterect, 0, RDW_UPDATENOW);
   //form1.refresh;
end;

Solution

Moving a panel should not take long. Also, to update the parent of the panel, a call to Form.Update should be just fine. To force a redraw of only a portion of a form, a call to InvalidateRect should do, as you have already done (try clearing the background though).

Possible reasons for the delay and/or smudging are:

you do some extensive own drawing in Form.OnPaint,
Anchors, Align, and/or Constraints of panel and/or form are contradicting or result in a chain-reaction (in this case, try DisableAlign and EnableAlign),
the form is not the immediate parent of the panel,
the panel is over some other control,
your updaterect variable is the new rectangle instead of the previous one,
something else in your code and/or calling routine,
any combination of the above.