Why is Wifi seen as bad for online gaming

Online games are not just sensitive to latency (measured by ping) and bandwidth (measured by speedtest, etc), but also to variance in latency, called jitter, as well as the overall reliability of the connection.

Wi-Fi is designed to work in a hostile RF environment. It must work despite microwaves, baby monitors, cordless phones, other Wi-Fi stations sharing the same frequencies. It deals with all this noise by checksum on each frame, and re-sending frames where the checksum is incorrect. This provides a higher likelihood of the data being delivered, but since some frames must be re-sent 1 or more times, some frames experience higher latency than others. Sometimes much more. If that frame carried information like "you moved to the left", it might not successfully be transmitted until you've been shot by a railgun.

Notably, most online games use the UDP transport. It differs from TCP in that if frames are lost or corrupted, they aren't automatically re-sent. This is good for games because if a frame arrives late, it might as well have not arrived at all. What happened 1 second ago isn't really important to a real-time game. By simply dropping failed or invalid frames, no time is lost processing old stuff which is no longer relevant to the gameplay.

However, Wi-Fi's retry behavior is directly contrary to this strategy. Even if you are using UDP, Wi-Fi sits at a layer below that. Even though it's detrimental to do so, Wi-Fi will retry failed and invalid frames.

In case you are wondering, the reason Wi-Fi adds this retry behavior, even though TCP has retry behavior of it's own, has to do with TCP's congestion avoidance algorithm. TCP assumes that when packets are dropped, it's because the network is congested, and so it slows down. This is a good assumption for wired networks where there's no interference, but not for Wi-Fi where interference is common.

When using radio waves to send signals between two computers, there is always a chance that interference will cause data to be corrupted along the way. This means that the machines sometimes have to retransmit data that was not received correctly at the other end - usually after a timeout.

Note that the same thing happens on wires, just MUCH less frequently, and the timeouts are shorter.

This means that WiFi introduces higher latency than if you are using a wire.

Latency is what kills gameplay. People teleporting, commands taking time to take effect, these are symptoms of high latency. And the more data is being sent, the higher the impact - so you might not notice it so much if you're walking down a corridor in a first-person-shooter, but as soon as you enter a room where there are lots of other players all engaged in a battle, everything will appear to grind to a halt and the next thing you know you're dead.

Use wires when you can, especially when gaming.

Much of wifi is in the 2.4 GHz band, which is known as an ISM band (one reserved for industrial, scientific and medical purposes). This band is subject to a lot of interference from things such as microwave ovens and Bluetooth devices, which also use 2.4 GHz.

When interference happens that disrupts packets in the air, a wifi device will retry to send the packets. This means packets will appear to take longer to reach their destination while the interference is occurring.

There is less interference at 5 GHz, so those devices should be affected less. However, they will still need to retry if interference does occur, which is usually not the case with a wired Ethernet interface going through a switch.