On the fiftieth position is not so important — what kind of account the position of the gamer. Therefore, update often "fair" rating makes sense for top50, where there is a cabin between the most ardent fans of the game in the first place.
further movement in the rating will not be sharp and will be a multiple of time of one game (the update interval of the rating of one player) and will not bounce on the average number of points in a single session. Thus, the ratings can be updated "on the eye" and to beat them in groups.
That's if the players are millions.
But at the same time online will be high hundreds, so I would suffer the optimization of this table.
And when at the same time to play there are thousands, will make refactoring code and databases, and maybe change the server.