procedure TForm1.FormActivate(Sender: T);
var
 Po :TPoint;
 a:TRect;
begin
a := ClientRect;
 with a do begin
  po := ClientToScreen(Point(Left,Top)) ;
  Left := Po.X;
  Top  := Po.Y ;
  Po := ClientToScreen(Point(Right,Bottom)) ;
  Right  := Po.X;
  Bottom := Po.Y;
 end;
  ClipCursor(@a);
end;

Speed test