rx 560x slower than integrated vega gpu on fedora 28

I regard the same issue as you on the Arch Linux (mesa-git, llvm-svn, linux 4.18.12 (even running 4.19.x or mainline 4.20rcx)). Stable linux kernel isn't, I would say, optimized for our gpu yet. Luckily, there is a temporary solution. I tried linux-amd-wip-git (drm-next-4.21-wip) which has the latest amd patches (there are also other kernels which have them like linux-drm-fixes-git (drm-fixes), depends on you which you pick). Many games are unplayable on the stable kernel and most of them run better on the integrated gpu than on the discrete one as you are also saying. linux-amd-wip-git makes perform discrete gpu better than integrated one and I also get e.g. ~30 fps gain on the World Of Warcraft via wine and gallium nine/dxvk. Some games perform even better, every I tried do. We just have to wait for merge of these patches to the stable linux. At the end, at least we know it will be fixed.

EDIT: I just tested the latest 5.0rc7, it works like aforementioned kernels. Seems it will be fixed in the 5.0.