Iterating over Image feature columns is extremely slow
State: open | Author: sohamparikh (NONE)

We are trying to load datasets where the image column stores `PIL.PngImagePlugin.PngImageFile` images. However, iterating over these datasets is extremely slow. 
What I have found:
1. It is the presence of the image column that causes the slowdown. Removing the column from the dataset results in blazingly fast (as expected) times
2. It is ~2x faster to iterate when the column contains a single image as opposed to a list of images i.e., the feature is a Sequence of Image objects. We often need multiple images per sample, so we need to work with a list of images
3. It is ~17x faster to store paths to PNG files and load them using `PIL.Image.open`, as opposed to iterating over a `Dataset` with an Image column, and ~30x faster compared to `Sequence` of `Image`s. See a simple script below with an openly available dataset.


It would be great to understand the standard practices for storing and loading multimodal datasets (image + text). 

https://huggingface.co/docs/datasets/en/image_load seems a bit underdeveloped? (e.g., `dataset.decode` only works with `IterableDataset`, but it's not clear from the doc)

Thanks!

```python
from datasets import load_dataset, load_from_disk
from PIL import Image
from pathlib import Path

ds = load_dataset("getomni-ai/ocr-benchmark")

for idx, sample in enumerate(ds["test"]):
    image = sample["image"]
    image.save(f"/tmp/ds_files/images/image_{idx}.png")

ds.save_to_disk("/tmp/ds_columns")

# Remove the 'image' column
ds["test"] = ds["test"].remove_columns(["image"])

# Create image paths for each sample
image_paths = [f"images/image_{idx}.png" for idx in range(len(ds["test"]))]

# Add the 'image_path' column to the dataset
ds["test"] = ds["test"].add_column("image_path", image_paths)

# Save the updated dataset
ds.save_to_disk("/tmp/ds_files")
files_path = Path("/tmp/ds_files")
column_path = Path("/tmp/ds_columns")

# load and benchmark
ds_file = load_from_disk(files_path)
ds_column = load_from_disk(column_path)
import time


images_files = []
start = time.time()
for idx in range(len(ds_file["test"])):
    image_path = files_path / ds_file["test"][idx]["image_path"]
    image = Image.open(image_path)
    images_files.append(image)
end = time.time()
print(f"Time taken to load images from files: {end - start} seconds")

# Time taken to load images from files: 1.2364635467529297 seconds


images_column = []
start = time.time()
for idx in range(len(ds_column["test"])):
    images_column.append(ds_column["test"][idx]["image"])
end = time.time()
print(f"Time taken to load images from columns: {end - start} seconds")

# Time taken to load images from columns: 20.49347186088562 seconds
```

Comments: Hi ! Could it be because the `Image()` type in dataset does `image  = Image.open(image_path)` and also `image.load()` which actually loads the image data in memory ? This is needed to avoid too many open files issues, see https://github.com/huggingface/datasets/issues/3985

Yes, that seems to be it. For my purposes, I've cast the column to `Image(decode=False)`, and only load the images when necessary, which is much much faster