hey @Matthieu, as far as i know the “fast” refers to the rust implementations of the tokenizers: tokenizers/tokenizers at master · huggingface/tokenizers · GitHub 2
there are bindings for python, so perhaps you can adapt the suggestion here 1 to your use case? e.g. download the tokenizer.json file for stsb and load the fast version as follows:
from transformers import RobertaTokenizerFast
tokenizer = RobertaTokenizerFast(tokenizer_file="tokenizer.json")

(i’m not super familiar with the stsb-xlm-r-multilingual model but am assuming it’s using the same tokenization strategy as XLM-R)