NEW YORK (BLOOMBERG) - iPhone users have begun racing to courthouses, infuriated by an Apple software update that slowed down the operation of their smartphones.

At least two groups of customers sued Apple on Thursday (Dec 21) over the deterioration of their phones' performance.

One group, in Chicago federal court, claims the software updates that throttle down the performance of iPhone 5, iPhone 6 and some iPhone 7s were intentionally put out to induce customers to buy newer iPhones such as iPhone 8 and iPhone X. That one retails for as much as US$1,149 (S$1,500) in the US.

A second group sued in Los Angeles federal court claiming Apple breached an implied contract in which the company agreed not to intentionally interfere with their usage or the speed of the device, and also interfered with their personal property.

Amy Bessette, an Apple spokeswoman, did not immediately respond to an emailed request for comment on the suits, sent before regular business hours in Cupertino, California.

Apple acknowledged this week that it issued a software update that slows down iPhones with dying batteries.

The fix was supposed to help people get more out of their ageing batteries, and "reduce occurrences of unexpected shutdowns", according to the company.

"Apple's iOS updates purposefully neglected to explain that its purposeful throttling down of older model devices and resulting lost or diminished operating performance could be remedied by replacing the batteries of these devices," the customers said in the Chicago lawsuit.

The customers in the Chicago suit are seeking unspecified damages. Those in the Los Angeles lawsuit want their old phones replaced and compensation for the loss of value and use of their phones.