Efficiency means output over effort. The only measure in which we consistently scored high over the years was time of possession. And no sane person would define a 'good' offense as 'one that holds the ball a long time.'

For an offense, the best measure of efficiency is more related to effectiveness - how many of my possessions resulted in points. An offense that scores on, say, half its possession is indeed effective, and the more possessions that result in TDs, the better your efficiency is.