I imagine Upgrading your wifi or network card may improve Stadia's gaming perfornance if your wi-fi or network card is very old and slow like some network cards having a max speed of 10 to 11 Megabit per second speed.
I wonder if most Internet Service Provider modems with built-in 802.11N or AC routers are good enough for playing games on Stadia at higher video frame rates like 60fps when streaming games to a computer with 802.11AC wi-fi card, or a 100Mbps or Gigabit Ethernet card.