Parkinson’s Law famously stated that work expands to fill the time available. That is why we say “If you want something doing, give it to a busy person.”
Non-busy people make themselves look busy by expanding the work to fill their available time. Busy people fit the work into their available resources, condensing the time to what’s left in their busy schedule.
As a manager, your job is to distinguish between the truly busy (and effective) and the work stretchers.