This seems the best place to ask.

I have a router with a 2.5gbps WAN port and 2.5gbps LAN port, with a 1.8gbps connection coming in via an ONT.

If I wire my PC by going from the routers 2.5 LAN port to my NIC, it'll negotiate at 1gbps - this is irrespective of using auto negotiate or setting the NIC to 2.5 in device manager.

If, using the same cable plus an additional (25 metre CAT6e), I go router to switch (Zyxel XGS1210-12) using the 2.5gbps port on both sides, then from the switch to my PC using 2.5gbps it'll negotiate at 2.5gbps.

I've largely concluded that the cables a bit naffed and am waiting on a replacement, but I can't figure out why going via the switch will make it work at full speed.

NIC drivers are all up to date, firmware on the router is all up to date. This presents between Windows and Linux (don't have another device with 2.5gbps handy to check with) so my only real conclusion is either the cable or something weird between the LAN port on the router and my PCs NIC negotiations