Fastest move?
Obvious recaptures is the best use, as the move will be cancelled if your opponent doesn't capture.
When I hover a piece during my opponents turn (all I have to do is release the mouse button to move it) the longest it seems to take is 0.5, but 0.2 is more like you were trying to make a premove, but the opponent moved too quickly, so you didn't get it down to 0.1
If someone literally made 6 moves in a row, all at 0.2, then that's really suspicious (any time a string of moves are played at the exact same rate it's suspicious, but 0.2 in particular is an odd one). But you were probably exaggerating a little.
A few times on chess.com, in blitz, I have been able to move in 0.1 seconds without premoving, but only because of hovering (and even then it usually takes a few tenths of a second). Realistically though, without hovering or premoves - a few tenths of a second is realistic (assuming you know what move to play if your opponent plays x). Of course, this is not counting the first move of the game; chess.com always begins the clock when White moves at move one (so technically this is the fastest).
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