Chapter: 387 (DBZ 193), P10.3, P13.1-4
Context: Goku just achieved Super Saiyan Grade III for the first time.
Gohan: “Ab-absolutely incredible power! You’ll definitely be able to defeat Cell like this!”
Goku: “I can’t win like this…Probably not…[ ] With my muscles swelled up like this, my power greatly increases, but it kills my speed. Huge power doesn’t mean anything if I can’t hit my opponent…And it uses up energy at too intense a rate. Balance-wise, regular Super Saiyan is best. I know that well enough…”
I realy doubt Goku would train Gohan to make him off balance while complaining about Grade 3's speed.
If you are faster, your hits are harder. Simply physics. I treat Cell's issue as a regular power up.