This is a small addon that tracks how fast you respond to ability cooldowns. Some minor configuration is required; you specify a GCD spell, one on the GCD but without a cooldown of its own, and enter what abilities to track.
There are tons of addons that track the GCD. You should look at those to see how they do it (hint: they track the cooldown of a spell that has no cooldown other than the GCD and cannot be silenced, such as Find Herbs).
Thanks, Phanx. I worked something out knowing that bit of info. It tracks abilities with actual cooldowns other than the gcd so far. I'll figure something out for the gcd-only abilities later. I mainly wrote this for me. I'm a fury warrior, so I'm not too interested in tracking the gcd that much (I can't think of a reason atm), but I imagine other classes probably would want to.
I updated this addon tonight so it can be used without a data broker display. Data broker support is still available. You can display your averages through the slash command: /rotlatency stats. I've only tested this on my fury warrior, tracking bloodthirst and whirlwind. I would love to receive some feedback whether it works for other classes.