iphone-主线程冻结半秒…为什么?

我有一个应用程序,是在石英背景画线。当用户在屏幕上移动手指时,应用程序开始绘图。

当上下文被保存到这里时,我会在移动时这样做

if (firstMove) // first movement after touchesbegan
  [NSThread detachNewThreadSelector:@selector(newThreadUNDO) 
     toTarget:self 
      withObject:nil];
  firstMove = NO
}

然后我有

- (void) newThreadUNDO {

  NSAutoreleasePool* p = [[NSAutoreleasePool alloc] init];

  [NSThread setThreadPriority:0.1];

  [NSThread sleepForTimeInterval:0.0];
  [self performSelectorOnMainThread:@selector(copyUNDOcontext) withObject:nil waitUntilDone:NO];

    [p release];
}

和

- (void) copyUNDOcontext {


  CGFloat w = board.image.size.width;
  CGFloat h = board.image.size.height;  
  CGRect superRect = CGRectMake(0,0, w, h);

  CGSize size = CGSizeMake(w, h);

  UIGraphicsBeginImageContext(size);
  CGContextRef new = UIGraphicsGetCurrentContext();
  // lineLayer is the layer context I need to save
  CGContextDrawLayerInRect(new, superRect, lineLayer);
  UIImage *imagem = UIGraphicsGetImageFromCurrentImageContext();

  [self saveTempImage:imagem :@"UNDO.png"]; 
  UIGraphicsEndImageContext();

}

问题是:一旦用户开始移动,新线程就被触发,但是即使这个新线程的优先级很低,主线程仍然会冻结大约半秒(可能是在保存文件时)。

为什么?

我怎样才能解决这个问题?

谢谢。