I think PCI network cards have its own processor built-into the PCI card which may make games run slightly faster on a slower computer if the network card no longer uses CPU resources from the CPU installed on your motherboard.
There are some network cards which have Gigabit Ethernet, so if your PC came with 100 mbps ethernet, using a gigabit network card may improve online gaming, and playing streaming games like Google Stadia.
There are some network cards which have Gigabit Ethernet, so if your PC came with 100 mbps ethernet, using a gigabit network card may improve online gaming, and playing streaming games like Google Stadia.