I think you guys need to first decide exactly what you mean by "fast player", then it should be easy to design the formula.

For example: To me, a fast player is a player who responds soon after I move. When I log on, it is usually my move in that game, and sometimes we are both online and we can make several moves in a row. If that's what you mean by "fast player" then just measure the average time between opponent's move and player's move in each game.