1x PCIE 3.0 has 8 Gbps raw speed - for 2.5Gbps duplex Ethernet you'll need 6~7 Gbps of raw link to CPU.

For 5Gbps and higher, you'll need another PCIE line - and SOHO motherboards are usually already pretty tight on PCIE lanes.

10GbE will require 4x3.0 lanes

Are motherboards commonly using PCIe 3.0 for onboard peripherals these days? I wouldn’t expect it to save them much money, but my PCIe knowledge is constrained to the application layer - I know next to nothing about the PHY or associated costs.