Ah I think this casting speed in DDO is really a bit annoying. I know they already speed it a bit but I honestly think that is not enough, especially if I bump on some mobs. I have no idea if you experiencing the same problem, having these kind of complain about this game feature as I am right now.
Honestly I am all here with you on this matter. I think in general trying to destroy some of the mobs is mission impossible and a bit pointless because they respawn too quickly for us mortal gamers. Destroying the dragon is quite a job even for the group. Although I am confessing that I had even bigger problems with casting speed in some other games. I think this is usual complain of any gamer - any casting speed is not enough! LOL