2012 Hyundai Accent GS: I bought my car brand new with only 9 miles on it. While driving two nights later, I couldn't get my car to accelerate past 3500 rpm's and it would go past 40 mph while in drive. I was able to use shift-tronic to get my car up to 60 mph, but it was really slow. Also, when I was at 60 mph I decided to put the car back into drive. When I did this, my car shifted into 4th gear and slowed down. After parking my car for the night, I woke up and took my car out again and had absolutely no problems. I don't have the problem now, but don't have a solution or reason for the isssue.
I would report this to the dealer and under warranty they should at least do a scan for any unresolved fault codes, I would get this done ASAP as if there is a problem it may let you down when you least expect it.