gluon-cv: Training speed of mxnet-ssd slows down?

 I have use record file(voc07+12) to train old-style ssd at a speed of 40 images/s ,The speed is about 25 images/s when  I try the new  train_ssd.py in gluoncv.
 I use rec dataset and  transform to replace origin file datasets in new ssd code. But when I set **num-workers=4** the gdata.DetectionDataLoader  failed ,while **num-workers=1** , It works but the speed is almost as  slow as original data reading method.
The  error infomation is as following:

Process Process-3:
Traceback (most recent call last):
  File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
  File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/home/deep/workssd/mxnet/incubator-mxnet/python/mxnet/gluon/data/dataloader.py", line 134, in worker_loop
    batch = batchify_fn([dataset[i] for i in samples])
  File "/home/deep/workssd/mxnet/incubator-mxnet/python/mxnet/gluon/data/dataset.py", line 126, in __getitem__
    self.run()
    item = self._data[idx]
  File "/home/deep/workssd/mxnet/incubator-mxnet/python/mxnet/gluon/data/vision/datasets.py", line 257, in __getitem__
  File "/usr/lib/python2.7/multiprocessing/process.py", line 114, in run
    record = super(ImageRecordDataset, self).__getitem__(idx)
  File "/home/deep/workssd/mxnet/incubator-mxnet/python/mxnet/gluon/data/dataset.py", line 180, in __getitem__
    return self._record.read_idx(self._record.keys[idx])
  File "/home/deep/workssd/mxnet/incubator-mxnet/python/mxnet/recordio.py", line 265, in read_idx
    self._target(*self._args, **self._kwargs)
  File "/home/deep/workssd/mxnet/incubator-mxnet/python/mxnet/gluon/data/dataloader.py", line 134, in worker_loop
    return self.read()
  File "/home/deep/workssd/mxnet/incubator-mxnet/python/mxnet/recordio.py", line 163, in read
    ctypes.byref(size)))
  File "/home/deep/workssd/mxnet/incubator-mxnet/python/mxnet/base.py", line 149, in check_call
    batch = batchify_fn([dataset[i] for i in samples])
    raise MXNetError(py_str(_LIB.MXGetLastError()))
MXNetError: [16:12:48] src/recordio.cc:65: Check failed: header[0] == RecordIOWriter::kMagic Invalid RecordIO File

It seems a multi-process problem with old rec file dataset?