"Men are fast."
The Korean word for men in the sentence is for an individual man, not men. Should be 남자들
It should be 남자들은 but you only use it to make an strong point. Korean dont really use plural very much
I think in this context it is man/men in general. It could be translated to Man is fast.
The 들 is optional depending on the context. In this case there is no context so both are correct.