More Fatigue In Earlier Episodes
Even though the Villains in the later episodes are supposed to be stronger. A difference between the earlier episodes to episodes Post-Frieza after the Super Saiyan Transformations, is Goku and even other characters seemed to stand less of a chance against the Villains, minus maybe Android 17&18.
Goku clearly held his own against a lot of characters and even if he would have injuries and recover by senzu beans or Dende, they seemed less serious. Goku seemed to stand less of a chance against Vegeta and Frieza than Cell or Buu. I'm sure some of you notice this.