Packet loss really, and the latency and jitter said loss can contribute to.
Radio waves go faster (speed of light) than through a medium (copper). Not that it matters at such a small scale, but it’s helpful to have a good picture of the elements at work here. The further you are from the receiving point, the more obstacles (matter) that can obstruct it. But in ideal conditions WiFi is better than most people think. Replicating those ideal conditions though…