AFAIK, there isn't solid info yet, or if there is it isn't widely known.
Using Skyedit, you can pull stats like this (very simplified list):
http://i.imgur.com/LwIAx.jpg
It looks like CastingDelay is casting time, but that's unconfirmed.
If you want, I could send you a csv or spreadsheet, but the best thing to do would be to download skyedit (if you're on PC).
http://github.com/uesp/skyedit
Thanks, I've taken a look at the spells in SkyEdit though, the CastingDelay (or casting time) you mentioned seems to be zero for single cast versions of destruction spells, and since you can't just sit there firing them as fast as you can tap the attack button, I think that number is just an extra delay that adds to however long the casting animation takes. I seem to remember some fields in the GECK's weapon editor that would tell you things like firing rate and reload time, taking animation into account, so hopefully the CK will provide similar information. I guess I could always just fire up the game and manually time how long it takes to cast the spells.

*Edit - Ok, I actually went in game and timed attack speed. One handed melee (not dual wield) I had about 1.17 attacks per second. I could cast fireball at about 0.87 attacks per second, no discernible difference for dual casting. If you're timing it perfectly, you might be able to squeeze a slightly higher aps out of destruction, but that was the best I could do.