Cooler absolutely slaughters his younger brother
In the movie Kame-Sen’nin says, “Ever since Goku returned from Planet Namek…” From this we can be sure that this is an event during the three years from when Trunks appeared to the arrival of the artificial humans.
— “Dragon Ball Daizenshuu 6: Movies & TV Specials” (p. 78)
Goku flat out says Cooler's ki is stronger than Freeza's when Cooler transformed. Goku in Movie 5 is also suggested to be much stronger than in the canon timeline because Cooler said he saw how he could've beaten Freeza after fighting Goku, who was still in base. Plus SSJ Goku strained himself immensely in the process of overpowering Cooler's Supernova, to the point of reverting back to base and passing out.