Hello lingbonet,
It really depends on how you define "best performance". If you want maximum bandwidth, you will want your memory. frequency to be 3200 MHz. For that, you will need to populate with 1 DIMM per Channel (DPC). You cannot achieve 3200 MHz with 2 DPC. If you want the best latency, then you'll want the memory frequency to be 2933 MHz, which can be achieved at both 1 DPC and 2 DPC. If "best performance" has a capacity parameter, then 2 DPC would also be the way to go.
So I know you don't want to hear it, but it really depends on what your final goal is.