If I can recall it took all the energy from all the Z fighters to defeat Broly and plus if you don't notice during the fight, Goku drained a part of Broly's power when his fist went in contact with his hand, you can clearly see the energy of Broly's consuming Goku. And plus King Kai immediately reacted to the South's Galaxy Destruction, it couldn't have been timelasped because King Kai had immediately watched the South Galaxy get erased and contacted Goku. If it was timelasped the Z fighters would've already been fighting Broly since the very beginning. It's still unknown by this day of how much power Goku actually wielded when he absorbed SSJ 1 Vegeta's, SSJ 1 Trunks, SSJ 1 Gohan, Full Powered Namek Piccolo (SSJ1 Sayian Level), and a piece of Broly's own power. SSJ1 power level can be increased during training.