Slowtwitch worked with Jeff Solomon of Tableau Software to present an interesting way to look at the top 10 Ironman performers in Hawaii from 1978 through 2008 and that allows you to see how the performances have trended through the years.
This graph is based on the data of the slowtwitch top 10 in Hawaii archive that Jordan Rapp has accumulated. What immediately sticks out of course are the early year of the event, but by 1982 the times very much settled down. Displayed below are the average time for each of overall, swim, bike and run of the top ten finishers against the fastest time (not winning time). You can also see the overall fastest time trend over all these years. The distance between the lines for a given time set is a measure of the strength of the field. Click on the radio button to either see male or female results and you can also change the date range to see just the races for any particular time period. You may also highlight just the swim, or the bike etc if that is a detail you are mostly interested in. Basically, just play with it a bit.