The only people who can address that is the carrier/service provider. They control the speed. Most providers have a cap on their speeds and/or data limits. So if yours is like some after you hit a certain amount you either have no connection at all, or you have a serious drop in speed. Usually it goes from 3G or 4G (depending on what you have) down to 2G.
Also, the location can be a large factor, in a few ways. If there are many other electronics around, either yours or your neighbors, that can interfere. Also they type of building and where in the building you are, as well as your location in comparison to the tower. Some of those you can change (like turning off electronics you aren't using if they are part of the problem. And some...