PCI-E lanes for consumers. Gigabit would saturate the PCI bus, but once you're on PCI-E you only need to give it 1 lane, usually off the chipset.

Servers had a reason to spend for the 10G, 25G and 40G cards which used 4 lanes.

There are 10 Gigabit chips that can run off of one PCI-E 4.0 lane now and the 2.5G and 5G speeds are supported(802.3bz).