Define awfully long time?
Visit the dealer for a lookover and make sure you take it for a ride with the tech. Drive another at the dealership to see if it does the same thing.
Could be a slow solenoid or something about to break. Could be low fluid.
And, are you sure its not the TC lock/unlock cycle vs. being in 3rd gear?
Form now on, when passing someone, learn to use the OD switch. This isn't a sports car. Don't expect it to have a brain reading instant shifting transmission.