I suspect it isn't the Phone, as much as it is the Carrier, and it's Bandwidth available to each Cellsite, the phone is connected to, and the number of Users, actively connected to that site, at any one time...... ......
Generally I would agree with you, except... We've had three generations of iPhone and six generations of iPod (including the last two generations of touch) and there is functionality that has nothing to do with being connected to or using the cellular network. If it were data related I could absolutely see the cell provider giving priority to certain types of devices, which I don't see as being that difficult to do. The problem comes in when you have a non-data consuming device (ie: non 3g/4g) that is wifi only and I know for darned sure that I'm not limiting any devices on my network. The device lags, slows down, freezes in applications and requires more constant reboots the closer it gets to the planned release of the new version (IOS or hardware). I am pretty confident in saying that apple includes some code in the iTunes updates that "talks" to the device and triggers this response, mostly because we had an iPod Touch that hadn't sync'd or been connected to iTunes in probably 8-10 months (one of the kids, nothing ever changes, etc, etc, etc). It was and had been running absolutely fine for that entire time. We updated iTunes (but NOT the IOS on the device) and did a sync/local backup and within a day it was acting up, stuttering, slowing down, not as responsive and needing to reboot at least once every two days (after a week). It showed the exact same over the air update that had been available for it (via wi-fi) for 6 months and the ONLY thing that had changed was talking to a new version of iTunes. We upgraded the IOS, just to get it out of the way, and it seemed to fix the problem for a while but this specific incident has been pretty much our normal experience with all IOS devices. Upgrade iTunes -> Connect device (for WHATEVER reason) -> Experience slowness until IOS upgrade or new device release. Again, I'm not talking about data transfer, I'm talking about usability of a device that is for all intents and purposes in airplane mode (yes, actually in airplane mode as well just to test).
That sounds like the new iTunes is trying to use Old Hooks in the IOS that just aren't there any more, and then reverting even older Subroutines that then VERY SLOWLY work around the issues. Now when you upgrade to the new IOS, the new Tunes can use the NEW APIs, in the later IOS, and operate the way it was intended to be operated. It is very common to leave legacy code in OSs to cover such contingencies, going back a few Generations. They do this for backward compatibility. ....