I have 1 stick of this RAM:
HyperX Fury 8GB DDR4-2666
Amazon.com: Kingston Technology HyperX FURY 2666MHz DDR4 Non-ECC CL15 DIMM 8 DDR4 2400 MT/s (PC4-19200) HX426C15FB/8: Computers & Accessories

It is supposed to be a good model of RAM, according to its average benchmarks, but I am only seeing my RAM score in the 7th percentile, shown here:

Screenshots on IMGUR of Memory PerformanceTest: Kingston Technology HyperX FURY 2666MHz DDR4 Non-ECC CL15 DIMM 8 DDR4 2400 MT/s (PC4-19200) HX426C15FB/8 - Album on Imgur

Screenshots are comparisons to other rigs with the same RAM, not global. The performance is ridiculously low.

It is running on this system:
Intel Core i7-6700K overclocked to 4.4 GHz
ASRock Z170M Pro4S motherboard

I found this list on Asrock's website: ASRock > Z170 Pro4S

It mentions this RAM:
DDR4 2666 8GB HyperX HX426C15FB/8 DS 2pcs v

So it should work in dual-channel mode (no surprise there). I have updated my BIOS firmware. It had no effect on RAM performance.

All of the timings and voltages are correct in the XMP profile and on Auto mode. Entering them manually did not change any performance.
I also have 2 modules in my computer now, and performance did not improve, so it has nothing to do with dual-channel mode or single channel mode.

I did some more research and found a post on Tweaktown that is relevant to this problem. According to the thread, I cannot fix this problem, and Asrock must fix their UEFI. Z170 Extreme4 poor performance of RAM with BIOS 1.80 & 2.10

I tried to contact the person who was fixing the problem in that thread (TSD Eddy), but I was unable to direct-message him on TweakTown (The message box was inactive when I went to send a message).

The problem on TweakTown was for a different Z170 motherboard, so I reasonably assume that this problem is shared by all of the Z170 motherboards, since their UEFI code-base is probably the same. (It would make sense for developers to reuse code between motherboards).

I emailed Asrock customer support and told them what the problem was (the UEFI is broken) and they pissed in my face.

your motherboard comes with latest bios version P 7.20) , if you have problems with the motherboard contact your vendor and ask for RMA

Asrock, are you going rectify this issue? You should have either NOT advertised this board as compatible with that RAM module, or you should have fixed the UEFI when someone brought up the issue over a year ago.