>I think it is not just looking at the percentage improvement. If 10% represents a few seconds, so what? But if 10% represents (for example) a few hours ...

During my study, in electronics tests, a result +- 10% would result in grade 2 (second best) no matter what was wrong. 10% of few hours is not remarkable.

Or the other way around, the 20/80 rule says 80% result with 20% effort. For the last 10% it's only worth it if there is a real difference - or you might miss Mars otherwise.
I normally don't get paid the last 20%. Guess what I care about less then 10%.