I am not really that interested in averages. I've encountered players that will maintain a high average, but still move slowly. Some people play games of one type rapidly, and of another type only when they're about to run out. Or they find the time to play 5 anti-backgammon matches in a day against their friend, and don't bother moving at their other games. High averages, but you don't really want to play them.

I propose the following statistic: given a game G a player has to move on, let T_G the time it has been the players turn. (So, if today is Dec 27, and his opponent has last moved on Dec 24, T_G is 3 days). Then let T_max the maximum of all T_G's of a player. A high T_max means the person is moving slowly in at least one game at the moment. If his T_max is close to when he last made any move, he's probably on vacation, or otherwise unable to play. If there's much difference between T_max and when he was last active, he has too many games running then he can manage.