Hello. My PC is hardwired to my router. A fresh boot nets a speedtest of ~945Mb/s which is what I expect. Shortly after, repeated tests start to drop significantly lower (sometimes ~90 Mb/s). If I reboot it's fine the first time again. Never had any kind of slowdown on my Z390 Hero XI which was my previous MB (same cabling).
Port always shows 1G and diagnostics never finds anything.
I put in a cheap PCIe ethernet card and it does not appear to exhibit this kind of slowdown.