Very true, you are only concerned about your flight, but if you know that Delta has 85% of their flights on time in Cincinnati, you know that you have a fairly good chance of being on time and can ease some of your fears. (Unless, of course, you're a pessimist and only focus on the 15% late.)
One thing to try is go to Delta's website and put in the flight number in the flight information section at the bottom of the page and click on "yesterday" or "today"... that will show you what time the flight was scheduled to arrive and actually arrived. This only works if the flight number is the same now as it will be when you travel, a lot of times the flight number or time can change periodically as schedules are adjusted.
When you book on American's website it gives the flight's historical ontime percentage. I haven't found that feature on Delta's website, and it sure would be nice when trying to plan connections.