How come a dataloader with 10 workers is slower than single process?

So I have a text file of about 3M training samples, with each line representing a sample. First I tried a single process to read samples line by line in a mini-batch style, then feed them to my model. Then I found that the GPU usage is just around 20%, so I guess the reading process is too slow. After I implemented a parallel version using DataLoader and expect it to have a prominent speedup, it turned out to be more than 2x slower than the single process version.
These are my code:

# an iterable object of a training data file
class SampleFile(object):

    def __init__(self, filePath):
        self.filePath = filePath

    def __iter__(self):
        with open(self.filePath) as file:
            while True:
                line = file.readline().rstrip("\n")
                if not line:
                sample = self.parseSample(line)
                yield sample

# DataSet object with (maybe) multiple data files
class TrainingSet(IterableDataset):

    def __init__(self, dataFilePath, workerNum):
        self.dataFilePath = dataFilePath
        self.workerNum = workerNum

    def __iter__(self):
        workerInfo =
        if workerInfo is None:
             # single process, just read the whole data
            return iter(SampleFile(self.dataFilePath))
            # read splitted data file generated by Linux `split` command
            workerNumLen = len(str(self.workerNum-1)) 
            suffix = (workerNumLen-len(str( * '0' + str(
            partFilePath = self.dataFilePath + suffix
            return iter(SampleFile(partFilePath))

Any one could give me a hint? Thanks