I would say alternator too. The cheap mans test is to start the car put on the headlights(and other electrical) and then disconect batery terminal(Negative, carefully) so the car is running purely on the alternator. If the lights dim down when battery is removed the alternator is not making sufficient power. You want to make sore your belts are tight, healthy and not slipping, that would also cause a drop in performance.

NOTE: It is safer to remove the negative cable on battery that way if it touches metal on the car it wont short.