procedure BetterScaleImage(AWidth, AHeight: Integer; sourceImage:Timage);
var
  srcImg, destImg: TLazIntfImage;
  cnv: TLazCanvas;
  interp: TFPBaseInterpolation;
begin
  if sourceImage.Picture.Bitmap = nil then Exit;

  srcImg := sourceImage.Picture.Bitmap.CreateIntfImage;
  destImg := TLazIntfImage.CreateCompatible(srcImg, AWidth, AHeight);

  try
    cnv := TLazCanvas.Create(destImg);
    try
      interp := TFPBaseInterpolation.Create;
      try
        cnv.Interpolation := interp;
        cnv.StretchDraw(0, 0, AWidth, AHeight, srcImg);

        sourceImage.Picture.Bitmap.LoadFromIntfImage(destImg);

      finally
        interp.Free;
      end;
    finally
      cnv.Free;
    end;
  finally
    destImg.Free;
    srcImg.Free;
  end;
end;                        

After loading an image in your TImage, simply call BetterScaleImage(width,height,TImage) to scale it better.
Under Windows this will be a world of difference. 

Enjoy!