Adding to what others say, I'd have him do tasks he's good at. Some people just don't have a higher gear. It sucks, but it's the way it is. If his speed is such a problem, having him not finish a task isn't a good solution. Maybe work with him to see what's taking so long. Part of being a leader of anything is equipping your team with the skills they need to do the job and allocating jobs that some people perform better than others.
I have one guy who is slow, but very thorough. He does a lot of the culling and stocking of visually appealing products like apples, avocados, etc. Another is quick, but not as thorough, so I prefer her to do salads and lunch meat, as quality is rarely, if ever, an issue.
Small changes like that can make a big difference in the grand scheme of things.