Dataset scan time is much slower than using native arrow
### Describe the bug

I'm basically running the same scanning experiment from the tutorials https://huggingface.co/course/chapter5/4?fw=pt except now I'm comparing to a native pyarrow version.

I'm finding that the native pyarrow approach is much faster (2 orders of magnitude). Is there something I'm missing that explains this phenomenon?

### Steps to reproduce the bug

https://colab.research.google.com/drive/11EtHDaGAf1DKCpvYnAPJUW-LFfAcDzHY?usp=sharing

### Expected behavior

I expect scan times to be on par with using pyarrow directly.

### Environment info

standard colab environment
Hi ! In your code you only iterate on the Arrow buffers - you don't actually load the data as python objects. For a fair comparison, you can modify your code using:
```diff
-     for _ in range(0, len(table), bsz):
-         _ = {k:table[k][_ : _ + bsz] for k in cols}
+     for _ in range(0, len(table), bsz):
+         _ = {k:table[k][_ : _ + bsz].to_pylist() for k in cols}
```

I re-ran your code and got a speed ratio of 1.00x and 1.02x Ah I see, datasets is implicitly making this conversion. Thanks for pointing that out!

If it's not too much, I would also suggest updating some of your docs with the same `.to_pylist()` conversion in the code snippet that follows [here](https://huggingface.co/course/chapter5/4?fw=pt#:~:text=let%E2%80%99s%20run%20a%20little%20speed%20test%20by%20iterating%20over%20all%20the%20elements%20in%20the%20PubMed%20Abstracts%20dataset%3A). This code snippet shows `datasets` code that reads the Arrow data as python objects already, there is no need to add to_pylist. Or were you thinking about something else ?