It will be losing 1-2% because that throughput is equal to PCIe 3.0 x16.
Aren't more lanes additional coming from the chipset?
I've never bought an amd motherboard, do they sometimes have 3rd party chipsets to offload things such as SATA from the CPU pcie lanes?
I don't think the asrock rack one I was looking at does.
https://www.asrockrack.com/general/productdetail.asp?Model=X570D4U#Specifications
Something worth considering with the latest graphics advances if your planning on using anything more than a single gfx card unless there are boards that retain x16 speed on the main slot even when other slot(s) are occupied.