AT&T has around 17 million customers with "unlimited data" plans.  And 17 million of those plans can be subject to data throttling.

AT&T stopped signing people up for these plans in 2010 and warned users then they would be subjected to throttling for reaching certain usage levels.  The threshold can be as low as 1.5 GB of data.  Moderate to heavy users of smartphones can reach that threshold within a week to ten days of their billing cycle.

AT&T customers can choose to purchase a limited access plan and have unthrottled access far beyond the 1.5 GB limit.  Many people see this as AT&T forcing their unlimited customers into adopting limited plans.

If you smartphone starts working so slowly it is rendered unusable, there is a good chance your carrier is throttling your connection.

For more see this source article.