Apple deliberately limits the performance of its iPhones when its battery deteriorates beyond a certain level to encourage users to upgrade, it has been claimed.
But Apple says it' actually trying to stop devices from randomly crashing as their batteries age.
Research carried out by Canadian tech company Primate Labs found iPhones exhibited a greater-than-expected drop in general functioning after the update from iOS 10.2.1 to iOS 11.1.2 in June 2017.
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Primate Labs founder John Poole says while updates can accelerate a decrease in performance, the drop is far more pronounced in iPhones.
"The difference between 10.2.0 and 10.2.1 is too abrupt to be just a function of battery condition," he told Geekbench.
"I believe, as do others, that Apple introduced a change to limit performance when battery condition decreases past a certain point."
When your battery deteriorates, performance does too as the flow between the battery and processor is restricted.
Mr Poole says limiting an iPhone's performance hides the fact the phone's battery is aging, which encourages users to replace the entire phone, rather than just the battery.
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"Users may believe that the slow-down is due to CPU performance, instead of battery performance," Mr Poole told Geekbench.
"[This will cause] users to think, 'my phone is slow so I should replace it' not, 'my phone is slow so I should replace its battery'."
In a statement, Apple said it does slow old devices down - in order to prolong their life.
"Lithium-ion batteries become less capable of supplying peak current demands when in cold conditions, have a low battery charge or as they age over time, which can result in the device unexpectedly shutting down to protect its electronic components."