Apple is still under fire after it recently revealed that it deliberately throttles iPhones' performance, and now the U.S. government is questioning the company about this practice.

It's no big secret that iPhones are not exactly cheap, and paying a small fortune for a smartphone only to end up with a sluggish performance has understandably angered many users.

Apple iPhone Slowdown

Apple has explained that it throttled the performance of older iPhones to make up for their aging batteries and avoid random shutdowns. Disgruntled customers, however, argued that this practice in fact aimed to force them to upgrade sooner than they would have if Apple hadn't slowed down the devices' performance.

A number of lawsuits against Apple claim the same and argue that the company should have been more transparent about the whole matter. If customers knew about this, they might have chosen to replace their handset's battery rather than purchase a new smartphone altogether.

Moreover, the company also received great criticism because it decided to come clean only after a Reddit user revealed evidence of CPU throttling. Otherwise, Apple might have never disclosed this practice. Apple apologized for not being more transparent and tried to make up for its mishap.

After stirring great controversy with its reveal, Apple discounted iPhone battery replacements from $79 to $29 to make amends. The measure might be too small to make up for the whole matter, however, and the company is now under investigation in its home country, as well as abroad.

Apple Questioned Over iPhone Slowdown

Just a couple of days after the French government launched an investigation into Apple's practice of throttling iPhones' performance, the U.S. government is now interested in learning more about this matter as well.

On Tuesday, Jan. 9, U.S. Senator John Thune sent a letter to Apple CEO Tim Cook, asking for more details about the company's policy to slow down older smartphones, The Wall Street Journal reports.

In the letter, Thune asked Apple to detail how it tracked customer complaints related to iPhone performance, and whether it considered offering rebates for customers who replaced their handset's battery at full price, before the $50 discount.

Thune also points out that Apple's solutions, especially the $50 discount on iPhone battery replacements, have sparked further criticism as some customers feel the replacements should have been free, considering the deliberate iPhone slowdown. The senator has asked Apple to offer a response to these questions by Jan. 23.