Why do US SIM cards slow down my Galaxy Note 3, and prevent system settings from opening? I have a problem that seems specific to US SIM cards. They cause the performance of my Galaxy Note 3 to tank, and I can't get into the system settings at all, without rebooting my phone into safe mode.

After a normal boot, when I tap the gear icon, the settings screen comes up briefly, but then vanishes. The same thing happens when I try to open the wifi connections screen, or anything else that calls up the system settings. And overall, the performance is so bad that I can barely enter text sometimes... it's like trying to run modern apps on a 5-year-old phone. But this phone is barely more than a year old.

And it's specific to the US. Whenever I arrive in a new country, I simply buy a local SIM card and stick it into my phone. Sometimes it costs me €10 or even £10 to get activated, but that's still a lot cheaper than international roaming rates. The US has always been a bit harder, because of its proprietary networks and the phone 'locking' craze, and phone companies trying to charge for the SIM cards themselves. I've found the solution to that last problem is to say 'that's outrageous' and start walking out of the phone shop—I always hear pleads of 'we can waive that' when I do. But there seem to be only two GSM networks, and a handful of resellers. They all seem to have this problem—T-Mobile, Simple Mobile, AT&amp;T. The problem goes away when I insert a SIM card I obtained in Australia, Asia or Europe, and I have no problems using my phone there.

I didn't think it was possible for SIM cards to run bloatware or anything like that, but I'm having a hard time coming up with other explanations. I suppose, perhaps, one of my apps tries to do something fancy when I'm in the US. Some US 'security' feature, perhaps?

Any ideas about how to solve this problem?
I believe part of the problem is twofold:


All the US phone providers I tried required manual APN configuration. (I've occasionally had to do this for providers in other countries, too.) Of course, configuring this required access to the settings menu. (I would probably have been able to configure this in safe mode, which allowed me access to the settings menu, if this had occurred to me.)
I have Avast Anti-Theft installed, which includes a feature that denies access to the settings menu, when the SIM card changes. Normally, when I change the SIM card, this doesn't inhibit network access (see #1 above), so other security features, such as the email I receive, makes it clearer that Avast Anti-Theft has kicked in, no matter how much of a doofus jetlag makes me.  I suppose this also highlights a workaround for Avast Anti-Theft. Whoopsie! 


I'm pretty sure this is the reason. Unfortunately, I ended up humouring AT&amp;T's opinion that my phone had malware, that somehow only kicked in when I'm in the US, so I wiped my phone, and thus don't know for sure. (I wonder if technical staff in other parts of the world would think malware might attack only their country!)