I think the logical explanation is quoted in the article: "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." However as one of the commenters has stated, as "there as so few models of the iPhone, software patches which will slow down a certain model of a phone to a point in which the user notices a general reduction of productivity is not acceptable."
Interesting theory though!
Is this not more of an issue around how often the devices themselves are released with higher spec and the new OS needing the faster processor/larger RAM to operate fully?
Take the BlackBerry Z10 an an example, it's over 18 months old has had 2 major OS updates since it was first released (not including the minor ones) and if anything it gets faster with every update. In that time there have been 3 different iPhones and 3 different iOS versions. Although the iPhone 4S is being muted as being iOS8 compatible I would bet that its going to be horribly slow...
Some more info about this topic comparing Apple's results with searches for 'Samsung Galaxy slow'
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