Saitama actively doesn’t go 100% from the start. His super moves are LITERALLY called the “Serious Series” because that’s what happens when he actually puts some effort in. I also sincerely doubt that the Serious Series is Saitama going at 100%, more like 10 or 20%. He’s at the level of strength where there’s less than fraction of a percentage of a rounding error between “strong enough to open a door” and “strong enough to cause even ‘Demon’ (city-level) threats to explode into chunks”.