Download GPU-Z and/or MSI Afterburner and use either of them to verify that the GPU and memory are running at full speed. If they are then I have a couple of questions:
1) What were you using before the R9 390?
2) Did you remove the display driver completely before installing the drivers for your new GPU?
3) What's the resolution of your monitor?
4) Mention some of the games you used to test the new card