Ethereum mining with the popular AMD graphics cards Radeon RX 480/580 and 470/570 could become unattractive. Cryptomining-Blog performed a benchmark test with blocks to be calculated ("DAG epoch"). This reveals a performance drop of up to 30% beyond the next 30 - 35 DAG epochs. This phenomenon occurred with both Claymore's and the original Ethminer mining software.
A change of the DAG epoch takes place every four to five days. Then the difficulty for the system and also the space requirement of the graphics memory increases. But beside the 4 GB-, also the 8 GB RX card types get problems with the performance. This should break up to 30 percent. Interestingly older AMD cards such as the R9 290(x), 390(x) and also Nvidias Pascal cards seems not to be affected.