It has nothing to do with the CPU's speed but with PCIe itself. It doesn't matter if you have the fastest super CPU in the world if the PCIe lanes are bottlenecking the entire system. Now keyword here is IF as we don't know yet if PCIe 4.0 will make any difference for ampere and RDNA 2 or if PCIe 3.0 16x is enough.
Only bottlenecking if you have to use them - if there is enough VRAM to hold everything (like 24 GB!) then it's all good: x16 link PCIe 3.0 = 32GB/s