I have tried to prepare ELI5 to train with T5, based on this wonderful notebook of Suraj Patil 4
However, when I run dataset.map() on ELI5 to prepare input_text, target_text, dataset.map is frozen in the first hundreds examples. On the contrary, this works totally fine on SQUAD (80,000 examples). Both nlp version 0.3.0 and 0.4.0 cause frozen process . Also try various pyarrow versions from 0.16.0 / 0.17.0 / 1.0.0 also have the same frozen process.
Reproducible code can be found on this colab notebook  4, where I also show that the same mapping function works fine on SQUAD, so the problem is likely due to ELI5 somehow.

More Info : instead of map, if I run for loop and apply function by myself, there’s no error and can finish within 10 seconds. However, nlp dataset is immutable , so I could not create a new key-value within the dataset directly ) .
I also notice that SQUAD texts are quite clean while ELI5 texts contain many special characters, not sure if this is the cause ?