Which video decoding speed should I expect from DALI?

Hi, I did few simple experiments and I had the feeling that I am getting a lower frame rate than expected. Can you help me understanding if I am doing things properly? Here some info:

• Docker image built on top of nvcr.io/nvidia/pytorch:22.11-py3 (hence nvidia-dali-cuda110==1.18.0)
• GPU Tesla V100-SXM2
• Video specs: 1920x1080@30fps, yuv420p, libx264, ~1.5Mbps (created with ffmpeg -i .... output.mp4 with default settings)

This is the code I am running:

import time
from pathlib import Path
from typing import Union

import nvidia.dali.fn as fn
from nvidia.dali import pipeline_def
from nvidia.dali.plugin.pytorch import DALIGenericIterator
from nvidia.dali.tensors import TensorListGPU
from tqdm import tqdm


def profile_dali_video_reader(profile_video_path: Union[Path, str]) -> None:
    @pipeline_def
    def video_pipeline(
        video_path: str,
        sequence_length: int = 1,
        step: int = -1,
    ) -> TensorListGPU:
        frames = fn.readers.video(
            name="reader",
            filenames=f"{video_path}",
            sequence_length=sequence_length,
            step=step,
            initial_fill=16,
            device="gpu",
        )
        return frames

    iterator = DALIGenericIterator(
        [
            video_pipeline(
                video_path=profile_video_path, batch_size=4, device_id=0, num_threads=1
            )
        ],
        ["frames"],
        reader_name="reader",
    )

    t_start = time.monotonic()
    for _ in tqdm(iterator):
        pass
    t_end = time.monotonic()
    print(f"Total time: {t_end - t_start:.2f}sec ({1000 / (t_end - t_start):.2f}FPS)")


if __name__ == "__main__":
    profile_dali_video_reader("tests/files/1000frames1080p.mp4")

Output is:

Total time: 203.96sec (4.90FPS)

Is it reasonable? Am I doing something wrong?