Wi-Fi lag

Wi-Fi lag is a drop in responsiveness during a game due to a slow or inconsistent connection while playing a game online. Wi-Fi lag was very common for Super Smash Bros. Brawl and is semi-common for Super Smash Bros. 4 and Super Smash Bros. Ultimate and for any games emulated on Project64k or Dolphin.

Overview
Wi-Fi lag can be a result of multiple things. The most common cause is a naturally slow connection such as a dial-up connection, which can degrade suddenly and without warning to the point of dropping inputs. A wireless connection, such as by a laptop, automatically causes lag on occasion, which affects the programs it is running. Other programs using the same connection, such as someone else using the router downloading a large file, will cause lag on all affected devices. The Nintendo 3DS specifically decreases power to wireless communications when in Power-Saving mode.

Wi-Fi lag is often confused with frame delay, though they are closely related. Frame delay is a delay between a button press and the corresponding action intentionally implemented by the developers to mitigate and hide Wi-Fi lag. Wi-Fi lag is not intentionally created by any involved party except under very specific, often malicious, circumstances like manually worsening a connection to disrupt gameplay.

In competitive play
Playing any online game with a Wi-Fi connection has always been a controversial and frowned upon practice, as its inherent flaws, inconsistencies and exploits could not only ruin the gameplay experience of the player, but everyone else playing with them. Wi-Fi lag was a very trodden-upon part of Brawl before the discontinuation of the game's online mode, and is often cited as one of its worst aspects. One reason that Brawl's lag differed from games with minimal visual Wi-Fi lag such as the Mario Kart series is that in such games a system can predict other player's positions while it waits for the next update – a half-second of lag is perfectly playable, and even if an incorrect assumption is made (e.g. opponent falls off an edge), it can be corrected with minimal disruption once discovered. In the Super Smash Bros. series, the system requires the exact position and action of every element of gameplay before it can do anything, and it is not possible to undo incorrect guesses; as exact consistency must be enforced 60 times every second, tiny amounts of lag can have a massive effect.

That being said, the Smash community was essentially forced to confront online play and its issues in 2020 due to the COVID-19 pandemic shutting down public events like offline tournaments. While all the aforementioned problems still existed and were only amplified with so many more people made aware, measures were created to like making players submit the strength of their online connection before entering a tournament and investigating alternative ways to play the game online. While Wi-Fi lag is impossible to rid completely, the community now has ways to deal with it moving forward.

Trivia

 * In Ultimate, some of the UI elements such as the match countdown will not be affected by the lag and will play normally.