Getting next item from IterableDataset took forever.
### Describe the bug

I have a large dataset, about 500GB. The format of the dataset is parquet. 

I then load the dataset and try to get the first item
```python
def get_one_item():
    dataset = load_dataset("path/to/datafiles", split="train", cache_dir=".", streaming=True)
    dataset = dataset.filter(lambda example: example['text'].startswith('Ar'))
    print(next(iter(dataset)))
```

However, this function never finish. I waited ~10mins, the function was still running so I killed the process. I'm now using `line_profiler` to profile how long it would take to return one item. I'll be patient and wait for as long as it needs. 

I suspect the filter operation is the reason why it took so long. Can I get some possible reasons behind this?

### Steps to reproduce the bug

Unfortunately without my data files, there is no way to reproduce this bug.

### Expected behavior

With `IteralbeDataset`, I expect the first item to be returned instantly.

### Environment info

- datasets version: 2.11.0 
- python: 3.7.12
Hi! It can take some time to iterate over Parquet files as big as yours, convert the samples to Python, and find the first one that matches a filter predicate before yielding it... Thanks @mariosasko, I figured it was the filter operation. I'm closing this issue because it is not a bug, it is the expected beheaviour.