"dataframe.groupby(level = index_name)" is much slow than "dataframe.groupby(col_name)"
I timeit, "groupby index" is almost 10 time slower than 'groupby column' , the index content is same as column for index is created by 'dataframe.set_index(col_name, drop=False)'.  Can "dataframe.groupby(level=index)" be improved ?
