60 FPS was the norm for older games, even as old as the NES. The only NES games that didn't run at 60 FPS were Technos games (Double Dragon, Kunio) or Micronics games (Ghouls N Ghosts, 1942). Most games run at 60 FPS under optimal conditions (as in no induced slowdown), only because the NES' hardware works at that speed. The Sega Genesis saw the rise of 30 FPS games, another example being Ranger-X, until it became a necessity to compromise framerate for barely passable graphics in 5th gen. 6th gen was a return to primarily 60 FPS games, then framerate took a hit again in the 7th.
As for Taz, I can't say there really is more going on in the overworld than in boss battles. In the overworld, Taz has to avoid spiders and rock monsters that randomly walk around, as well as deal with water spouts. There are a few helpful items scattered around, as well as the boss battle. The boss battle is a couple of platforms where a truck runs back and forth. I don't think the enemies would chew up that much CPU time, unless they found it absolutely necessary to keep track of all enemies on the current stage.