Herd immunity kicks in when a large fraction of the people you encounter were already infected, giving you fewer chances to infect new people. So incubation time doesn’t matter there (though certainly it doesn’t help, for other reasons.)
> Herd immunity kicks in when a large fraction of the people you encounter were already infected
Alternatively, immunized people.
I think there is a momentum effect if the percentage of infectious people gets high enough. Where you overshoot the static herd-immunity threshold. If you can keep the number of infected below 10% then you don't see it. At least that's what I saw when I tried to model it. (Take that as you will).