The carrier limits the speed of the connection. A single LTE connection is 5-15 mbps. Some carriers support multiple simultaneous connections when the capacity permits (and iPhones support this), so you can get higher than that at unpredictable times. But they also throttle the speed of the connection when network load is high. And with some unlimited plans the carrier limits the speed further, typically to 3 mbps. That’s why unlimited is not necessarily a good option.
The size of an iOS major update is around 3 GB. A 5 mbps (megabits/sec) connection becomes 0.5 megabytes/sec. Some simple math shows that a downloading an update at that speed will take 1.5 to 2.5 hours, with overhead allowance. And if the connection is interrupted even briefly it has to start over. So there’s a practical reason not to update over cellular, also.
What you should do is get the speedtest.net app and see what your actual speed is.
BTW, one way around the limit on updates over cellular is to get a cellular hotspot device (“Mifi” or equivalent) and add it to your account. These appear to be Wi-Fi networks to the phone, and can be used for updates. They are also more efficient than the personal hotspot on your phone. I’ve gotten as high as 90 mbps with the ZTE model I use. This is also useful if you don’t have broadband in your area.
