It is hard to figure out without all the information.
If your groundspeed went from 490 to 515 as you say, it is indeed hard to imagine where you lost 5 mins. It certainly wouldn't have been in the climb to 390.
Personally I doubt that the flight actually took five minutes longer than it would have if you had stayed at FL330.
Basically i think the answer lies in where you are getting the information from that you lost 5 mins. I think it is not the right place.
Framer.
PS was FL350 and FL 370 not available?