*As said the problem is DDR4 bandwidth.
Your iGPU in that APU will take just about all the bandwidth you can give it. Even 2x8GB 3200MHz CL15 is probably not maxing out the bandwidth it can utilize.
**Without getting super detailed:
a) TWO STICKS in Dual Channel gives 2x the bandwidth from DDR4 memory to the CPU
b) roughly speaking that APU can benefit from up to and probably more than 2x 3200MHz CL15
c) 1x2666 is about 42% of that bandwidth (though performance doesn't scale exactly like that)
So...
I strongly recommend you:
#1 - ensure motherboard BIOS is the latest
#2 - run a couple BENCHMARKS and write down the CPU, GPU and Total scores (like Timespy or Futuremark/3DMark series) immediately prior to upgrading
#3 - get another...