I've seen a lot of folks saying their performance dropped using DX12 mode in Rise of the Tomb Raider, so I figured I would run the test. It appears, at least in Nixxes implemention of DX12, that for better performance you'll need a 6-core Intel CPU.
System Specs:
CPU: i7-5820k@4.5ghz
GPU: R9 290x @ 1100mhz(core)/1400mhz(RAM)
RAM: 16GB DDR4 2133 in Quad-Channel
EDIT - Resolution: 2560x1080
Mountain Peak (DX11): Minimum: 36.89, Maximum 108.22, Average: 68.66
Mountain Peak (DX12): Minimum: 48.73, Maximum 78.73, Average: 66.48
Syria (DX11) Minimum 12.68, Maximum 87.69, Average 63.47
Syria (DX12): Minimum 30.49, Maximum 98.22, Average 61.48
Geothermal Valley (DX11): Minimum 13.40, Maximum 76.7, Average 63.06
Geothermal Valley (DX12): Minimum 23.51, Maximum 164.59, Average: 60.76