Hi @Chae-Ho_Yim,
For this number of materials, it would probably be easier to run:
with MPRester() as mpr:
docs = mpr.query({'material_id': {'$in': all_mat_ids}, ['structure', 'material_id'])
Where all_mat_ids is a list of material ids of interest.
However, the line IStructure.add_oxidation_state_by_guess(structure) is probably what’s taking the most time, not the API retrieval.
Best,
Matt