Unimaginable super slow iteration
In every iteration you load the full "random_input" column in memory, only then to access it's i-th element.

You can try using this instead

a,b=dataset[i]['random_input'],dataset[i]['random_output']
### Describe the bug

Assuming there is a dataset with 52000 sentences, each with a length of 500, it takes 20 seconds to extract a sentence from the dataset……？Is there something wrong with my iteration?

### Steps to reproduce the bug

```python
import datasets
import time
import random

num_rows = 52000
num_cols = 500

random_input = [[random.randint(1, 100) for _ in range(num_cols)] for _ in range(num_rows)]
random_output = [[random.randint(1, 100) for _ in range(num_cols)] for _ in range(num_rows)]


s=time.time()
d={'random_input':random_input,'random_output':random_output}
dataset=datasets.Dataset.from_dict(d)
print('from dict',time.time()-s)
print(dataset)


for i in range(len(dataset)):
    aa=time.time()
    a,b=dataset['random_input'][i],dataset['random_output'][i]
    print(time.time()-aa)

```

corresponding output
```bash
from dict 9.215498685836792
Dataset({
    features: ['random_input', 'random_output'],
    num_rows: 52000
})
19.129778146743774
19.329464197158813
19.27668261528015
19.28557538986206
19.247620582580566
19.624247074127197
19.28673791885376
19.301053047180176
19.290496110916138
19.291821718215942
19.357765197753906

```

### Expected behavior

Under normal circumstances, iteration should be very rapid as it does not involve the main tasks other than getting items

### Environment info

- `datasets` version: 2.19.0
- Platform: Linux-3.10.0-1160.71.1.el7.x86_64-x86_64-with-glibc2.17
- Python version: 3.10.13
- `huggingface_hub` version: 0.21.4
- PyArrow version: 15.0.0
- Pandas version: 2.2.1
- `fsspec` version: 2024.2.0