I'm assuming this is a case of unusually slow 4G, and not fast 3G? I travel frequently and I've noticed that sometimes my 4G speeds are very slow even when I have good signal strength, always in rural areas. I'm not sure what explains that.
The mast that your phone communicates with doesn't always have lots of fiber bandwidth. Sometimes it's only connected to other masts using microwave links (which could be outdated).