1. Memory usage if you have kept stuff stored on it. Did resetting to factory delete everything too?
2. The fact that various apps loaded on the phone have gone through many updates while the underlying OS has stayed the same.
3. Flash memory (used as the only storage mechanism on smart phones) has a limited number of times that a particular bit cell can be rewritten without degradation and failure. After a long enough use the phone has less memory to use and also has to work around gaps in that memory.
If you could go back to the original OS with the original version of all of the installed apps and with a complete wipe of the memory, you might get back to original performance, or you might not if it is mainly a memory issue.
i'm voting on tired flash memory. SSD's (high quality ones) have extra
data blocks in them, and wear leveling chipsets to keep the drive from
potholing... but... cellphones, with a life span engineered for 18~24 months
or so, just have what they have, and it gets worn with use... and this
stuff is engineered to be *just enough* to get you to the next phone
after this one.
as much as i love a good "evil empire" plot, if any cellphone maker
intentionally built in a "cripple function", i can think of three things
happening, right off the top of my head....
his competition, reverse engineering his product, would discover
this, and spread the information far and wide to damage the the
competition's credibility.
can you imagine what samsung would pay for proof that the iphone
has a death code embedded in the firmware?
the second thing would be the resulting class action lawsuit that
would crop up.
but the third thing that would show up first...... would be some
el33te haqu3r (code for 17 year old geek in des moines, or beijing)
figuring out how to kill every iphone on the planet, and doing it.
i've muttered darkly about the "black void" theory after some of
my stuff has died.
i had TWO identical fujitsu laptops, and within 60 days of the
warranty expiring, so did the screens.
within about 10 days of each other. and they had hugely different
usage amounts. makes one mutter and peer around for black
helicopters, and global conspiracies.
it's just extremely complex stuff made as cheaply as possible
in a hugely competitive environment.