(see previous question id no. 20191964)
I found the problem. It was a BIOS setting, but I am unsure what it does. Please help if you know what this does! Bios is AMIBIOS:
'CPU In Order Queue' was incorrectly set to '1-level'.
Set this to '4-level'.
Memory benchmark doubled in transfer rate; CPU performance has also increased by at least 2x.