二级域名走OSS下载非常慢

server {
  listen          80;
  server_name     down.domain.com;


  location /{
     secure_link $arg_s,$arg_e;
     secure_link_md5 key$arg_e;
     if ($secure_link = "") {
          return 403;
     }
     if ($secure_link = "0") {
          return 403;
     }
        expires 30d;
        etag off;
        proxy_pass http://domain.oss-cn-beijing-internal.aliyuncs.com/;
        proxy_set_header Host "domain.oss-cn-beijing-internal.aliyuncs.com";
        proxy_set_header Referer "key";
   }
}


走主域名时速度很快，有人知道原因不？