PCIe is full duplex. And there's no requirement for ethernet ports to be able to do full tilt. Even with a 1x PCIe 3.0, a 10G port will be much much better than a 2.5G one.
(But PCIe 3.0 of course is from 2010 and isn't too relevant today - 4.0, 5.0, 6.0 and 7.0 have 16/32/64/128 Gbps per lane respectively)