`.to_json` is extremely slow after `.select`
Hi ! It's slower indeed because a datasets on which `select`/`shard`/`train_test_split`/`shuffle` has been called has to do additional steps to retrieve the data of the dataset table in the right order.

Indeed, if you call `dataset.select([0, 5, 10])`, the underlying table of the dataset is not altered to keep the examples at index 0, 5, and 10. Instead, an indices mapping is added on top of the table, that says that the first example is at index 0, the second at index 5 and the last one at index 10.

Therefore accessing the examples of the dataset is slower because of the additional step that uses the indices mapping.

The step that takes the most time is to query the dataset table from a list of indices here:

https://github.com/huggingface/datasets/blob/047dc756ed20fbf06e6bcaf910464aba0e20610a/src/datasets/formatting/formatting.py#L61-L63

In your case it can be made significantly faster by checking if the indices are contiguous. If they're contiguous, we could pass a python `slice` or `range` instead of a list of integers to `_query_table`. This way `_query_table` will do only one lookup to get the queried batch instead of `batch_size` lookups.

Given that calling `select` with contiguous indices is a common use case I'm in favor of implementing such an optimization :)
Let me know what you think
## Describe the bug
Saving a dataset to JSON with `to_json` is extremely slow after using `.select` on the original dataset.

## Steps to reproduce the bug
```python
from datasets import load_dataset

original = load_dataset("squad", split="train")
original.to_json("from_original.json")  # Takes 0 seconds

selected_subset1 = original.select([i for i in range(len(original))])
selected_subset1.to_json("from_select1.json")  # Takes 212 seconds

selected_subset2 = original.select([i for i in range(int(len(original) / 2))])
selected_subset2.to_json("from_select2.json")  # Takes 90 seconds
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: master (https://github.com/huggingface/datasets/commit/6090f3cfb5c819f441dd4a4bb635e037c875b044)
- Platform: Linux-4.4.0-19041-Microsoft-x86_64-with-glibc2.27
- Python version: 3.9.7
- PyArrow version: 6.0.0

`.to_json` is extremely slow after `.select` 
 ## Describe the bug
Saving a dataset to JSON with `to_json` is extremely slow after using `.select` on the original dataset.

## Steps to reproduce the bug
```python
from datasets import load_dataset

original = load_dataset("squad", split="train")
original.to_json("from_original.json")  # Takes 0 seconds

selected_subset1 = original.select([i for i in range(len(original))])
selected_subset1.to_json("from_select1.json")  # Takes 212 seconds

selected_subset2 = original.select([i for i in range(int(len(original) / 2))])
selected_subset2.to_json("from_select2.json")  # Takes 90 seconds
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: master (https://github.com/huggingface/datasets/commit/6090f3cfb5c819f441dd4a4bb635e037c875b044)
- Platform: Linux-4.4.0-19041-Microsoft-x86_64-with-glibc2.27
- Python version: 3.9.7
- PyArrow version: 6.0.0
 
 Hi ! It's slower indeed because a datasets on which `select`/`shard`/`train_test_split`/`shuffle` has been called has to do additional steps to retrieve the data of the dataset table in the right order.

Indeed, if you call `dataset.select([0, 5, 10])`, the underlying table of the dataset is not altered to keep the examples at index 0, 5, and 10. Instead, an indices mapping is added on top of the table, that says that the first example is at index 0, the second at index 5 and the last one at index 10.

Therefore accessing the examples of the dataset is slower because of the additional step that uses the indices mapping.

The step that takes the most time is to query the dataset table from a list of indices here:

https://github.com/huggingface/datasets/blob/047dc756ed20fbf06e6bcaf910464aba0e20610a/src/datasets/formatting/formatting.py#L61-L63

In your case it can be made significantly faster by checking if the indices are contiguous. If they're contiguous, we could pass a python `slice` or `range` instead of a list of integers to `_query_table`. This way `_query_table` will do only one lookup to get the queried batch instead of `batch_size` lookups.

Given that calling `select` with contiguous indices is a common use case I'm in favor of implementing such an optimization :)
Let me know what you think