While traveling this past week, and staying in a hotel with awful wifi, I tried working with my MacBook Air tethered to my iPhone 6. The network performance seemed almost as bad as the hotel wifi, even though presumably I had a good 4G connection.
By chance, I tried tethering to my iPad mini, connected to the same Vodafone 4G network, and the performance was much better.
Running SpeedTest on both devices revealed terrible performance on the iPhone, compared to the iPad:
As you can see, download speeds of 37 megabits per second on the iPad, but only 0.5 megabits per secondfor the iPhone. And a similar story with upload speeds.
Both devices are connected to the same Vodafone 4G network. Both are running iOS 8.4. The only difference I can see is the carrier profile settings — version 20.0 on the iPad and 20.1 in the iPhone.
After resetting my iPhone to factory settings, which allowed me to revert to carrier settings version 20.0—a process, btw, that took a few hours, since a full restore-from-backup is required—the network problems went away, and I again had 40ms ping times, and 50+ Mbit of download and upload.
A few days later, though, getting tired of having to cancel the carrier update alert, I allowed it to update the carrier settings to 20.1 and immediately experienced the networking problems again. I noticed, however, that if I activate a VPN over 4G, everything is fine.
A friend of mine has an iPhone 6 with the 20.1 carrier setting, and his iPhone’s 4G networking works fine. So it seems to be some problem with version 20.1 settings specifically on my iPhone.
Unfortunately, I can’t find any way to revert carrier settings, so I’m afraid I’m going to have to reset and restore the phone again. 🙁