async function testDownloadSpeed() {
  const downloadUrl = '/testfile.bin'; // 必须是 ≥5MB 的文件
  const startTime = performance.now();

  try {
    const response = await fetch(`${downloadUrl}?cache=${Math.random()}`, {
      cache: 'no-store'
    });

    const blob = await response.blob(); // 确保完全读取数据
    const endTime = performance.now();

    const fileSizeMB = blob.size / (1024 * 1024);
    const duration = (endTime - startTime) / 1000;
    const speedMbps = ((fileSizeMB * 8) / duration).toFixed(2);

    console.log(`📥 下载速度：${speedMbps} Mbps`);
    return {
      speed: speedMbps,
      fileSizeMB: fileSizeMB.toFixed(2),
      duration: duration.toFixed(2)
    };
  } catch (e) {
    console.error('下载测速失败:', e);
  }
}