I was wondering if anyone had any issues or knows what the problem might be with my wifes phone? It is a Samsung Galaxy, originally a US TMobile phone. We have TKS month to month service, and everything on her phone seems to work fine, but her data is EXTREMELY slow and at times doesn't work at all. WiFi seems okay, though she may be having issues with that too.

I tried my SIM card in her phone and vise versa to be sure it's not something with her service, and the problem stayed with the phone, and her card worked perfect in my phone. I've looked through her APN settings and compared them to mine, and even changed some things and cannot find any reason her data would be so slow. TKS said it is something with the phone, which is apparent but I am trying to determine if the phone is garbage and we should buy a new one or if there is maybe something with her settings that can be changed to help with the issue? It's hard to explain what the problem is, but websites load extremely slow and it won't stream YouTube videos at all. Most of the time it won't even connect to the YouTube app, it just says "Error Loading Page" or something to that effect. There have been a couple times (though very few) where YouTube has worked.

Anybody have any ideas? I can give a quick rundown later of what her APN settings are to see if maybe something isn't setup right, though they seem to mirror mine fairly closely. 

I forgot to mention, sometimes WhatsApp messages won't even go through. Her data is just THAT slow and it seems to be an in and out issue with her WhatsApp and internet, but streaming videos are a constant issue. 

Have you tried searching the S4 forums at xda-developers.com?

http://forum.xda-developers.com/search.php?searchid=294110490

Dunno about the Galaxy S4 but my Nexus 5 is sold in two different versions because LTE frequency bands are different here than in the US and it cannot be fixed with software or settings. If you have 4G/LTE turned on, see if dropping back to 3G in your mobile network settings helps.