delphi – 有什么方法可以加速TPNGImage上的SaveToStream？

我有一个函数将TBitmap(我绘制)转换为TPng Image,然后将其保存为流,因此其他方法可以使用它.使用Png是因为它为报告输出创建了较小的图像(excel,html).问题是SaveToStream似乎花费了太多时间,比将TBitmap转换为TPngImage或使用带有png的TStream多15倍.这是代码：

var
 BitmapImage: TBitmap;      
 PNGImage: TPngImage;
 PngStream: TStream;        
begin
  // draw on BitmapImage
  ...
  PNGImage := TPngImage.Create;
  PNGStream := TMemoryStream.Create;
  Try
     PNGImage.Assign(BitmapPicture.Bitmap); // Step 1: assign TBitmap to PNG
     PNGImage.SaveToStream(PNGStream);  // Step 2: save PNG to stream
     WS.Shapes.AddPicture(PNGStream,PNGImage.Width,PNGImage.Height); // Step 3: Add PNG from Stream to Excel
  finally
     PNGImage.Free;
     PNGStream.Free;
  end;
...

这是通过70000张图片测试的,以下是时间：
第1步：7秒

第2步：93秒

第3步：6秒

为什么保存到Stream这么慢？有什么建议可以优化吗？

使用Delphi XE7

编辑

这是带有简单bmp的示例(MCVE),它将转换为PNG,然后保存到流中.只是为了另一个验证,我添加了SaveToFile,当然需要更长时间,但它保存到磁盘,所以我认为可以接受.

img1.bmp为49.5KB,保存的PNG为661字节.
链接到img1.bmp = http://www.filedropper.com/img1_1

TMemoryStreamAccess = class(TMemoryStream)
  end;

procedure TForm1.Button1Click(Sender: TObject);
var BitmapImage:TBitmap;
  PNGImage:TPngImage;
  PNGStream:TMemoryStream;//TStream;
  i,t1,t2,t3,t4,t5,t6: Integer;
  vFileName:string;
begin

  BitmapImage:=TBitmap.Create;
  BitmapImage.LoadFromFile('c:tmpimg1.bmp');

  t1:=0; t2:=0; t3:=0; t4:=0; t5:=0; t6:=0;

  for i := 1 to 70000 do
  begin

    PNGImage:=TPngImage.Create;
    PNGStream:=TMemoryStream.Create;
    try

      t1:=GetTickCount;
      PNGImage.Assign(BitmapImage);
      t2:=t2+GetTickCount-t1;

      t3:=GetTickCount;
      TMemoryStreamAccess(PNGStream).Capacity := 1000;
      PNGImage.SaveToStream(PNGStream);
      // BitmapImage.SaveToStream(PNGStream); <-- very fast!
      t4:=t4+GetTickCount-t3;

    finally
      PNGImage.Free;
      PNGstream.Free
    end;

  end;

   showmessage('Assign = '+inttostr(t2)+' - SaveToStream = '+inttostr(t4));
end;

type
  TMemoryStreamAccess = class(TMemoryStream)
  end;

var
  BitmapImage: TBitmap;      
  PNGImage: TPngImage;
  PngStream: TMemoryStream;        
begin
  // draw on BitmapImage
  ...
  PNGImage := TPngImage.Create;
  Try
    PNGImage.Assign(BitmapPicture.Bitmap); // Step 1: assign TBitmap to PNG
    PNGStream := TMemoryStream.Create;
    try
      TMemoryStreamAccess(PNGStream).Capacity := ...; // some reasonable value
      PNGImage.SaveToStream(PNGStream);  // Step 2: save PNG to stream
      WS.Shapes.AddPicture(PNGStream,PNGImage.Height); // Step 3: Add PNG from Stream to Excel
    finally
      PNGStream.Free;
    end;
  finally
    PNGImage.Free;
  end;
  ...