DDR4 is slow. You cannot use it for ethash or CN-7
Etash nope, but CN-7 you can at ease. 700H/s for stock ryzen 2600X If I remember correctly what I read here. that's with lower than 3000MHz mem. Best mem can achieve 4200MHz, giving 107.52 GB/s bandwith.
CPU miners use L3 cache, not DDR ram. On pure DDR ram(or on cpu with little amount of cache) it will be slow