If you are a chip design house designing graphic processors, and you find that a certain percentage of your chips coming from the foundry does not perform on specification, you have 2 choices. Either you discard them, or you redefine the specification to make it a different product. This particular way is quite usual and AMD shows the latest incarnation of this practice. The AMD Radeon HD 6790 is a cut-down Barts chip targeting the mainstream segment. HT4U.net compares an AMD reference board and a branded version from Sapphire.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips