Saitama follows the rule of gag. Whether he is late or not, whether he is fast or not, depends on if the plot demands it. Shall I remember yall how he struggled to catch a mosquito while running after him in one of the first chapters? Even if he were the reincarnation of One Piece Zoro, his speed is so insane he can reach out for the other end of Garou's portal before the latter could cross more than half a body through it; yet he was late to Deepsea King fight. He tanked a GRB and a Nuke right in front of his face and was unfazed, yet Genos' light "stunned" him and needed to adapt to the lava in Orochi's fight like anyone does when they enter a pool. He learnt Garou's martial art by copying him perfectly, both his "fist" he used on the monster form and his time travel, yet he couldn't copy Suiryu's fighting style and thought of them as just "cool moves".
It doesn't have to make sense powerscalingwise, it just must happen for either a gag to be made or the plot to progress without him instantly solving everything in one punch.