When you cross your limit in your service plan, your browsing speed is reduced(throttled) as per your plan which you took this is called APN throttling.
You can apply few hacks on device for high speed unlimited data use. These hacks require tweaking with the APN configuration on the device. Normally there are some private APN(s) at enterprise level which do not have throttling configured. Once, a device configures to these APN, the plan speed degrade can be avoided, temporarily. But these are not full proof solutions as networks are slowly getting smarter and smarter.
