use msi afterburner and see what is the clock of gpu and memory when mining of bad gpu. some reason to slow down is overheat and powerlimit control of gpu.
with afterburner software you can set fan settings to control the temperature and set powerlimit control. also to reduce the power consume undervolt to -70 -80 mV of GPU core and downclock the gpu core clock to 1100 Mhz.
i have the same problem with three new xfx rx480 gpus, but after slowdown the speed when mining crashed with grey or black screen. That was warranty issues and company change all of three cards.