using map on loaded Tokenizer 10x - 100x slower than default Tokenizer?
Hi @wumpusman 
`datasets` has a caching mechanism that allows to cache the results of `.map` so that when you want to re-run it later it doesn't recompute it again.
So when you do `.map`, what actually happens is:
1. compute the hash used to identify your `map` for the cache
2. apply your function on every batch

This can explain the time difference between your different experiments.

The hash computation time depends of how complex your function is. For a tokenizer, the hash computation scans the lists of the words in the tokenizer to identify this tokenizer. Usually it takes 2-3 seconds.

Also note that you can disable caching though using
```python
import datasets

datasets.set_caching_enabled(False)
```
This could total relate to me misunderstanding particular call functions, but I added words to a GPT2Tokenizer, and saved it to disk (note I'm only showing snippets but I can share more) and the map function ran much slower: 

````
def save_tokenizer(original_tokenizer,text,path="simpledata/tokenizer"):
    words_unique = set(text.split(" "))
    for i in words_unique:
        original_tokenizer.add_tokens(i)
    original_tokenizer.save_pretrained(path)

tokenizer2 = GPT2Tokenizer.from_pretrained(os.path.join(experiment_path,experiment_name,"tokenizer_squad"))

train_set_baby=Dataset.from_dict({"text":[train_set["text"][0][0:50]]})
````

I then applied the dataset map function on a fairly small set of text:

```
%%time
train_set_baby = train_set_baby.map(lambda d:tokenizer2(d["text"]),batched=True)

```


The run time for train_set_baby.map was 6 seconds, and the batch itself was 2.6 seconds

**100% 1/1 [00:02<00:00, 2.60s/ba] CPU times: user 5.96 s, sys: 36 ms, total: 5.99 s Wall time: 5.99 s**

In comparison using (even after adding additional tokens): 
`
tokenizer = GPT2TokenizerFast.from_pretrained("gpt2")`

```
%%time
train_set_baby = train_set_baby.map(lambda d:tokenizer2(d["text"]),batched=True)

```
The time is 
**100% 1/1 [00:00<00:00, 34.09ba/s] CPU times: user 68.1 ms, sys: 16 µs, total: 68.1 ms Wall time: 62.9 ms**

It seems this might relate to the tokenizer save or load function, however, the issue appears to come up when I apply the loaded tokenizer  to the map function. 

I should also add that playing around with the amount of words I add to the tokenizer before I save it to disk and load it into memory  appears to impact the time it takes to run the map function. 



using map on loaded Tokenizer 10x - 100x slower than default Tokenizer? 
 This could total relate to me misunderstanding particular call functions, but I added words to a GPT2Tokenizer, and saved it to disk (note I'm only showing snippets but I can share more) and the map function ran much slower: 

````
def save_tokenizer(original_tokenizer,text,path="simpledata/tokenizer"):
    words_unique = set(text.split(" "))
    for i in words_unique:
        original_tokenizer.add_tokens(i)
    original_tokenizer.save_pretrained(path)

tokenizer2 = GPT2Tokenizer.from_pretrained(os.path.join(experiment_path,experiment_name,"tokenizer_squad"))

train_set_baby=Dataset.from_dict({"text":[train_set["text"][0][0:50]]})
````

I then applied the dataset map function on a fairly small set of text:

```
%%time
train_set_baby = train_set_baby.map(lambda d:tokenizer2(d["text"]),batched=True)

```


The run time for train_set_baby.map was 6 seconds, and the batch itself was 2.6 seconds

**100% 1/1 [00:02<00:00, 2.60s/ba] CPU times: user 5.96 s, sys: 36 ms, total: 5.99 s Wall time: 5.99 s**

In comparison using (even after adding additional tokens): 
`
tokenizer = GPT2TokenizerFast.from_pretrained("gpt2")`

```
%%time
train_set_baby = train_set_baby.map(lambda d:tokenizer2(d["text"]),batched=True)

```
The time is 
**100% 1/1 [00:00<00:00, 34.09ba/s] CPU times: user 68.1 ms, sys: 16 µs, total: 68.1 ms Wall time: 62.9 ms**

It seems this might relate to the tokenizer save or load function, however, the issue appears to come up when I apply the loaded tokenizer  to the map function. 

I should also add that playing around with the amount of words I add to the tokenizer before I save it to disk and load it into memory  appears to impact the time it takes to run the map function. 


 
 Hi @wumpusman 
`datasets` has a caching mechanism that allows to cache the results of `.map` so that when you want to re-run it later it doesn't recompute it again.
So when you do `.map`, what actually happens is:
1. compute the hash used to identify your `map` for the cache
2. apply your function on every batch

This can explain the time difference between your different experiments.

The hash computation time depends of how complex your function is. For a tokenizer, the hash computation scans the lists of the words in the tokenizer to identify this tokenizer. Usually it takes 2-3 seconds.

Also note that you can disable caching though using
```python
import datasets

datasets.set_caching_enabled(False)
```