About the time I was heading off to college I was introduced to an interesting phase. I am not entirely sure where I heard it, but I believe it was from my Aunt. Throughout her career she worked at IBM, Dun & Bradstreet, and Hewlett Packard. The saying went something like this: "If you need something to get done, give it to someone who is very busy."
As a kid who had never had a white-collar job before I didn't understand it. It still flies in the face of conventional thinking; surely you should give a task that needs to get done to people who have free time to do it. Only as my career has advanced am I beginning to understand the inherent wisdom in the phrase.
The first layer of understanding this wisdom is Parkinson's Law: "work expands to fill the time allotted." In the context of delegating an important task that needs to be completed; if you give the task to someone with enough free time the task will take longer. Delegate accordingly.
The second layer of understanding this wisdom, you'll discover, is that some people aren't interested or motivated. It is incredibly rare to see a skilled and ambitious person twiddling their thumbs waiting for something to do between 9 and 5. That said, staring at the ceiling trying to figure out the right decision to make counts as working for many of us. If something is urgent, out of the ordinary, or outside someone's comfort zone (and they haven't agreed to a stretch goal) you may reconsider delegating to this person.
Make no mistake, this is not about "a lack of trust". You trust the person to do their job, if you don't they shouldn't work there. But oftentimes things that need to get done lie on blurred lines.