The folks over at Computerbase.de have took it into their hands to study exactly how much of an impact >(if any) would activating HDR on a 4K panel affect performance cross different hardware configurations. Supposedly, HDR shouldn't impose any performance penalty on GPUs that were designed to already consider that output on a hardware level; however, as we know, expectations can sometimes be wrong.
Comparing an AMD Vega 64 graphics card against an NVIDIA GeForce 1080, the folks ate Computerbase arrived at some pretty interesting results: AMD hardware doesn't incur in as big a performance penalty (up to 2%) as NVIDIA's graphics card (10% on average) when going from standard SDR rendering through to HDR rendering. Whether due to driver-level issues or not is unclear; however, it could also have something to do with the way NVIDIA's graphics cards process 4K RGB signals by applying color compression down to reduced chroma YCbCr 4:2:2 in HDR - an extra amount of work that could reduce frame rendering. However, it's interesting to note how Mass Effect Andromeda, one of the games NVIDIA gave a big marketing push for and that showcased HDR implementation, sees no performance differential on the green hardware.I also seem to remember some issues regarding AMD's frame time performance being abysmal - looking at Computerbase's results, howveer, those times seem to be behind us.