load_from_disk takes a long time to load local dataset
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
```

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 :)
load_from_disk takes a long time to load local dataset 
 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 :) 
 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
```
