There is definately bottleneck somewhere!
This bottleneck is related to screen resolution, as opensource driver does not reduce the fps on par to closed source.
Maybe some huge data copy function/s that is unoptimized or unaccelerated? Maybe closed source copies data in single SIMD every frame, but opensource calls on demand?
Do amd developers have some profiling software, some software to monitor cpu load, memory bus, pcie bus, gpu load, gpu memory controller load, disk io, software io(like kernel ring 0/1 changes per (milli)second, time percentage and overall the cpu spends - in kernel, driver or elsewhere or idles)?