I think a good objective measure of frequency of slow periods and their seriousness is the number of duplicate posts that happen. A good running setup will post quickly enough that people do not feel the need to press the submit button again. That really only shows a fraction of the problem though because most threads have far more passive readers than posters. So you know that there are serious problems, and need to do something about them, when duplicate posts start showing up with consistency
It would be interesting to see if one could plot duplicate threads over time to see if you can find any long-term patterns.