Side-by-side tests have shown that iOS 9 makes older iPhones perform more slowly, lending weight to long-held suspicions that Apple might deliberately make its older devices slow down to encourage users to upgrade to newer models.

Tests carried out by vloggers iAppleBytes, show that the latest mobile operating system, iOS 9.0.1 creates a "noticeable amount of slowdown with the new update", The Guardian reports.

Running the new software, the iPhone 4S, 5 and 5S models take longer to start up, have slower camera performance and take longer to launch and switch between apps, the vloggers show.

Some analysts have suggested that running updated software is more complicated for older devices, but this does not seem to be true on Android, The Guardian notes.

"Some updates – Google’s Android Lollipop in 2014 for example – focus on performance and optimisation, and can actually improve the speed of the smartphone or tablet they are installed on. This does not appear to be the case with iOS 9."

Last year, research in the US lent further weight to the deliberate slowdown theory. The study, compiled by Harvard University PhD student Laura Trucco, analysed worldwide searches for "iPhone slow" and found that the search term spikes massively around the time of new phone releases, The Times reports.

The study compared those results with similar searches for one of Apple's main rivals but found that the term "Samsung Galaxy slow" was unaffected by new releases from Samsung.

Writing for the New York Times, Sendhil Mullainathan, a professor of economics at Harvard, said that the results were "striking".

"Wouldn't many business owners love to make their old product less useful whenever they released a newer one?" Mullainathan wrote. "When you sell the device and control the operating system, that's an option".

On the other hand, conspiracy theorists might also see an economic reason for Google to hide search results displaying peaks of concern about the speed of Android operating systems built by Google and used in Samsung smartphones.

Mullainathan notes that the research does not prove that Apple has done anything wrong. "No matter how suggestive," he says, "this data alone doesn't allow you to determine conclusively whether my phone is actually slower and, if so, why."

He said that there are other plausible explanations for why an older model iPhone may slow down. The latest version of the Apple operating system, iOS, is always tailored to the newest device and may therefore not work as efficiently on older models.

Also, artificially slowing phones comes with many problems: "First, the legal risk," Mullainathan says. "Second, competition and consumer rationality should combine to thwart this strategy. All a competitor needs to do is to offer a smartphone that doesn't become a brick as quickly, and more people should buy it."

Apple did not respond to requests from The Times for comment.