Claymore's ZCash/BTG GPU Miner v12.6 gives only 28 H/s on RX550 under Windows 7. Is that OK? Also it designates card as "gfx804" instead of "Lexa" which is the name of the chip.
As others have stated, the RX550 is not very good graphics card for mining. However, it should be capable of much more than 28 h/s. My guess is that Claymore did not code in support for the "Lexa" chip in version 12.6. You may have to try another miner if he doesn't plan to add support for it in his future releases. Also, just to make sure nothing is wrong on your end, please verify that you have the correct drivers installed. You can refer to the first post of this thread for a list of the best drivers to use.