When a router is used for routing between multiple LAN segments and broadband internet, it's quite possible to saturate your LAN link to the point where one is negatively impacting the other. In other words, if you're moving a lot of data between LAN and DMZ, other LAN clients aren't going to get anything like a gigabit out of that cherished FTTH link.
The most effective solution would be either:
A) Use two routers, and have your DHCP server push any local routes in addition to the default route; or
B) Buy a router and a switch with at least one trunk port of 2.5Gbps or better each. (Why new consumer routers with a single NBase-T port always label that the "WAN" port is more than I can guess. The average home/SOHO user is much more likely to need it on their LAN.)
Any other options? I've toyed with bonding (a bit iffy on consumer devices that use VLANs to split a single PHY over a switch) and noodled around using multiple LAN interfaces with proxy ARP, but haven't come up with a solution I like. And of course it's academic if the router hasn't got more than a 1Gbps internal link between the host and switch, which is probably the case with a majority of consumer routers.
I seldom have to move entire filesystems between local network segments and when I do a temporary dip in WAN performance is probably not going to affect anything. But I'm curious if anyone has a solution to this I'm not seeing.