IME, there's always a cut-off point beyond which you should stop the iOS upgrades. We might want "the shiny", Apple may promise it's faster, with better battery life - but that never seems to apply to older devices. The iOS version it comes pre-installed with is generally fine, as is typically the next major upgrade. After that it's a bit of a lottery.
I've been caught out a couple of times, never again.
It's not just a matter of the iOS being written/optimised for the latest devices with faster CPUs and more memory - it's also a QA issue. The iOS version preinstalled has always received a thorough testing, while 2/3 year old devices would be a much smaller part of the test matrix. Hence there's a greater likelihood of bugs when upgrading iOS on older devices.
I've been caught out a couple of times, never again.
It's not just a matter of the iOS being written/optimised for the latest devices with faster CPUs and more memory - it's also a QA issue. The iOS version preinstalled has always received a thorough testing, while 2/3 year old devices would be a much smaller part of the test matrix. Hence there's a greater likelihood of bugs when upgrading iOS on older devices.