You are correct. They are a function of time, but that's only one side of the coin.
If you have 305 dex and fully hasted you will proc on average, 2 times a minute. If you have a 20 dly weapon, you will swing 60 times in 1 minute. if you are going to proc 2 times a minute off of 60 swings, the game calculates that you have roughly a 3% chance to proc per swing.
If your delay is 70, you will swing roughly 8 times per minute (with no haste). So to get the same number of procs, your adjusted chance to proc is roughly 25% per swing.
Now, slow yourself by 60%. (again no haste or haste item) your adjusted delay is now over 100. Less then 6 swings per minute. You see where this is going?
Now take rampage (the only way you can use an earthshaker to AE). That forces a swing against every mob in range (double, triple and flurry can trigger as well, so at LEAST one swing per mob).
If you make one swing at every mob instantaneously and you have a 30-50% chance to proc per swing, the over time factor does not come into play.
http://www.thesteelwarrior.org/forum/sh ... shakerpage
For another example (and a better explanation by far)