Rollback netcode
Carry on from a guess about the opponent's input, then rewind and recompute when the real one turns up.
Tony Cannon built GGPO in 2006 because fighting games are indecently sensitive to delay — a single 16 ms frame decides whether a combo lands. The method assumes the opponent is doing whatever they did last frame, simulates straight ahead, and when the real input arrives a few frames later restores the game state and replays those frames with correct data. The player perceives a small visual hitch rather than a delay on every button press.
The requirements are demanding. The simulation must be fully deterministic, the entire game state must be saveable and restorable several times a second, and the replay has to fit inside one frame. The alternative — delay-based netcode, which waits for everyone — dominated Japanese titles for years and accounts for much of the reception Street Fighter V received. Rollback is now treated as a baseline requirement in the genre.