load_from_disk takes a long time to load local dataset
Hi !
Can you share more information about the features of your dataset ? You can get them by printing `my_dataset.features`
Can you also share the code of your `map` function ? It is actually just the tokenized `wikipedia` dataset with `input_ids`, `attention_mask`, etc, with one extra column which is a list of integers. The `text` column is removed during tokenization.

```
def add_len_and_seq(example):
    end_idx = example['input_ids'].index(SEP)
    example['actual_len'] = end_idx-1
    seq_len = len(example['input_ids'])
    

    example['seq'] = [PAD_ID] + [np.uint8(example['some_integer'])]*(end_idx-1) + [PAD_ID]*(seq_len-end_idx)
    
    return example
```
 Is `PAD_ID` a python integer ? You need all the integers in `example['seq']` to have the same type.
Does this work if you remove the `np.uint8` and use python integers instead ? yup I casted it to `np.uint8` outside the function where it was defined. It was originally using python integers. Strangely, even when I manually created `np.arrays` of specific `dtypes`, the types in the final `dataset_info.json` that gets written are still `int64`.

Update: I tried creating lists of `int8`s and got the same result. Yes this is a known issue: #625 
We're working on making the precision kept for numpy :)
To specify the precision of the integers, currently one needs to specify the output features with `.map(..., features=output_features)` Do you know what step is taking forever in the code ?
What happens if you interrupt the execution of the dataset loading ? After a synchronous discussion, we found that the cache file sizes have an enormous effect on the loading speed: smaller cache files result in faster load times. `num_proc` controls the number of cache files that are being written and is inversely proportional to the individual file size. In other words, increase `num_proc` for smaller cache files :)

Maybe this can be highlighted somewhere in the docs.
I have an extremely large tokenized dataset (24M examples) that loads in a few minutes. However, after adding a column similar to `input_ids` (basically a list of integers) and saving the dataset to disk, the load time goes to >1 hour. I've even tried using `np.uint8` after seeing #1985 but it doesn't seem to be helping (the total size seems to be smaller though).

Does anyone know what could be the issue? Or does the casting of that column to `int8` need to happen in the function that writes the arrow table instead of in the `map` where I create the list of integers?

Tagging @lhoestq since you seem to be working on these issues and PRs :)