Wi-Fi lag, which is very often confused with frame delay, is a drop in the FPS (frames per second) of 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 for games emulated on Project64k or Dolphin. Wi-Fi lag can be a result of multiple things:
Wi-Fi lag was a very trodden-upon part of Brawl prior to 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 had a massive effect.