Obviously, a single trigger is potentially much faster.
But, as others have pointed out, the gun must be properly set up to realize this advantage.
I used to use a double trigger Stoeger. It was a good gun. I now use a highly tuned SKB –– in fact I have a number of them. My match times went down by almost 30 seconds when I made the switch from the Stoeger to the SKB. Not all of that time is due to the single trigger, but much of it is!
Get a good single trigger gun from a good CAS gunsmith. Don't mess around with other solutions. Believe me, doing it right will save you lots of time and money in the long run.
Remember, ALL OF THE TIME IS IN THE SHOTGUN. That's where you want to spend your money.