All smartphone batteries degrade over time so it is not unusual for Apple to be slowing down the performance of its older phones to help prolong their battery life, according to experts.
But some owners prefer faster performance instead.
They were reacting to Apple's statement on Wednesday - in response to a claim by an app company at the beginning of this week - that it had introduced a software feature last year to prevent phones from unexpectedly shutting down.
Apple said the move was meant to reduce power demands - which can have the effect of slowing the processor - when a phone's battery is having trouble supplying the peak current that the processor demands.
Currently, affected models are iPhone 7, iPhone 6, iPhone 6s and iPhone SE, with other models potentially receiving the same feature in the future.
Ms Karissa Chua, a consumer electronics consultant at Euromonitor International, said it was common for the battery, speed and performance of all smartphones to degenerate over time with every update to the operating system and apps.
She said this was because "with every upgrade to the system, more processing power from the phone's hardware is required and this will place a greater strain on the battery life of the phone."
Professor Subodh Mhaisalkar, executive director of the Energy Research Institute at Nanyang Technological University, said that rather than goading iPhone users to upgrade by making their older phones slower, Apple's move in fact helped consumers by allowing batteries to be used for a longer period.
"(But) it's a trade-off. When your battery is degrading, you have the choice of either living with a slightly slower operation of the phone, or changing the battery," he said.
Some iPhone users, who said they would prefer a faster phone, wanted Apple to be more transparent with the effects of its updates.
Civil servant Jovita Chua, 27, said: "If it's a trade-off to fix something, then users should know of any 'side effects' that are involved from the start.
"The solution should not be at the expense of the user's experience, which might be affected if the phone slows down."
Mr Chan Choy Kee, 62, who works in aviation management, noticed his three-year-old iPhone 6 Plus slowing down about a year ago.
"It's not right," he said, referring to Apple's throttling of the processor. "Why not just offer to replace the battery instead?"