There are a lot of issues with implementing something like this.
How do you handle time a game is paused looking for replacements? Should a GM be 'penalized' for that? We don't track data to remove that time from the equation.
How about special games like the winter blitz? That will bring the average down without cause relative to their normal game speed.
Also, any metric like this will have 2 effects.
1, It will cause GMs to race ahead, trying to reduce their number. This is not necessarily a good thing for game Quality. Any time you track and publish a metric, this happens. Human Nature. So you need to use metrics that will push forward helpful behaviors (like the metrics we currently have on the site.)
2, players will use that metric to 'punish' slower GMs. Our GMs are all volunteers, and besides those who are so poor then need to be referred to the moderators, I will vote against anything that could cause a GM to feel bad about a game (s)he ran for any other reason.
Games get delayed for all kinds of reasons, and I think such a metric would be unfair, and counterproductive.
That said, suggestions like this are Great! Please make more and dont get discouraged by those that dont find wide support.