Yes, because defensive Pokemon takes 2 second pause between each attack animation, their energy generation decreases proportionally. However, most defending Pokemon receive most of their energy from damage taken.
Defending Pokemon receive double hp bonus, and for every damage taken, they also receive half an energy. In effect, defenders have an hp bar worth of energy to work with in a full battle. A beefy Snorlax, for example, could fire off almost 4 Hyperbeams from hp loss alone.
This means for most defenders, their primary source of energy is through taking damage, not quick moves.
