Sen. John Thune (R-SD), chairman of the Commerce, Science, and Transportation Committee, said in the letter that Apple has failed to act with enough transparency. Among other questions, the letter asks whether the company will issue updates in the future that slow device performance, and whether it tracked consumer complaints about performance issues.

After the discovery of the slowdown issues, which Apple said compensated for problems with degrading battery performance, the company offered a discount on battery replacements,…