The delay is to prevent trap stacking, allowing someone to set (ie) 50 traps and lure a strong enemy through them effortlessly.

As of 0.67, the cool down will be reduced to 6s (one trap activation per round). The DCs have also been increased as they were pretty awful.

A good tactic is to lay several traps, lure an enemy to one trap and then fight them for the rest of the round, and then lure them on to the next trap etc.

Traps cannot be on the same level of balance as spells. Traps are essentially unlimited, where as spells are not, and can be prepared in mass before the fighting even occurs so you can avoid having to cast - you can just run away. With the new timer of 6s, one trap per round, I think trap damage will be more than fair, assuming you prep a proper amount before battle. You have to remember as well, the 3 traps you have access to are the newbie traps, much more powerful ones are to come. Your traps will be hitting for the 100s in your higher levels.