I know people say things like this, but how much of it is true and how much is perception.

For example on an old phone the chances are you are still using the latest versions of apps, which are now coded for faster models. So trying them on your 3GS means they use more memory, crash more and slow down the OS. Taking a vanilla install I don't think you would see much difference in common tasks.

you are right, almost certainly a lot of it is perception based on the poor quality of apps in a legacy environment.

some of it is certainly OS features though. the framerate on animations can be quite poor - like slideshow poor - when launching an app, swiping at notifications or bringing up the status bar. the onscreen keyboard on the 3G is something i remember as being particularly terrible from when I had access to such a device... it would lose input entirely in some situations. some of this is probably also apps in the background, but its also the OS management of those apps...

ultimately though i think the real thing is that apple don't test in real world usage cases (like you describe) - or have a very low quality bar on what is acceptable on older devices. if the way most users use their OS makes it slow then it just is the OS developers fault imo...

EDIT: i would point out that as an app developer its definitely not the speed of the hardware - its the difference between os versions that i see - nothing else changed.

You can't assume from your limited experience with a 3G running iOS 4.2 that an iPhone 3GS running iOS 6 or an iPhone 4 running iOS 7 will be as bad.

Those issues were caused by the anemic processor and 128mb of RAM in the original 3G. The 3G was arguably unusable with iOS 4.2 but the same situation really hasn't repeated itself with more recent devices.

thank you. i did say precisely this, that the current fears are measurably unjustified, but obviously i didn't make clear enough that this was what i meant. :/