In general use though slack has an even stronger connotation of e.g. slacking off and not doing anything useful with the time.

Alternatively, ensuring you have enough slack in the schedule is, at least for some tech leads and project managers, an essential tool to enable meeting deadlines.

(So, I suppose using "slack" in a positive sense by project management, while probably still being considered a pejorative thing by non technical management or beancounters...)