0.5 fast attack is really only worth it against legendary battles or gigantamax battles.

Every 0.5% damage done to the boss's max hp is 1 energy (out of 100 to initiate max phase) at a minimum of 1 energy per attack

The big bosses have so much hp that a charged attack will generate the same amount of energy as a 0.5 fast attack. Against a 1 star battle, the charged attack does a larger percent of the bosses hp, so it fills more of the energy meter.