In live chess settings there's an option to turn on "premove"
Premoving allows you to make a move on your screen during your opponent's turn. The website remembers this move, and will play it for you immediately after your opponent moves (only if the premove is legal).
On chess.com premoves cost 0.1 seconds, so you can make 8 moves in 0.8 seconds.
On lichess premoves cost 0 seconds, and in some (ridiculous) cases I've made 40 or 50 moves while only losing 1 second off my clock there.
Top human speed is probably something like 0.5 seconds per move, but with a little lag it's possible for both players to premove. This is especially true on chess.com since you can "stack" up to 8 premoves. In other words you can make 8 moves in a row during your opponent's turn, and they will be played one after the other (if they're legal). A practical example of stacking premoves is when promoting a pawn or forcing checkmate.
A few months back, I signed up to a paid membership as chess.com. I've since cancelled it, as there seems to be an abundance of cheating. I usually play the quick fire 3 min games, and the number of people who make a ridiculous number of moves in under a second towards the end is crazy. I've just had a game where someone made 8 moves in 0.9 seconds. In comparison, I took 3.6seconds to make the same number of moves. The human body is not capable of reacting that quickly.
Does anyone have any idea how ppl do this?