用HttpGet频繁访问固定URL, 速度越来越慢

URL url = new URL(urlStr);
httpConn = (HttpURLConnection) url.openConnection();
httpConn.setDoInput(true);
httpConn.setDoOutput(false);
httpConn.setConnectTimeout(60 * 1000);
httpConn.setReadTimeout(150 * 1000);
httpConn.setRequestMethod("GET");

final BufferedInputStream bis = new BufferedInputStream(httpConn.getInputStream());
final DataInputStream dis = new DataInputStream(bis);

final FileOutputStream fos = new FileOutputStream(new File(saveFolder, name));
byte[] buffer = new byte[1024];
int length;

我用这种方式去下载某个固定URL的文件，发现下载的速度会越来越慢．刚开始还能比较快的下载下来，然后就慢慢的降下来了．我用的多线程来做的这件事情，有谁知道这是什么原因吗?