Always presume the best

If this person is slow, there must be a solid reason. Just forget what you think it is: go there and ask why. Address the issue in a positive way, as if you both want to find a solution.
You’ll be surprised at how many unseen factors determine a slow performance in otherwise dedicated workers.
Are they overwhelmed with busywork? Did they get assigned to difficult tasks? Are they perfectionists? Do they lack some equipment?
It’s going to be much, much easier to come up with a solution once you can get a grasp of what the real problem is.

Stimulate their sense of belonging

Many of said problems can be solved with practical means: clearer priorities, updated hardware and software, smaller subtasks, etc. All simple strategies that work well in the short term.
To obtain a long-term improvement in performance, however, motivation is key. Why should the perfectionist choose quantity over quality? Why should the specialist struggle with assignments out of their field?
The answer is simple: do it for the team. Belonging and relatedness are key motivators, especially effective when single tasks have a low intrinsic value for the worker. Feeling useful, even crucial to the team’s success, can inspire them to do better - no matter how dull the current task is.

Focus on growth

When properly supported, people will invest their efforts to grow their talents, striving for better and better results.
Becoming quicker and more efficient can be a challenge - but constant, sustainable challenges are a surefire path for improvement.
When discussing ways to streamline your slow worker’s output, use growth as an incentive. Introduce working faster as an actual way to acquire new skills and advance their career.

None of this would work, of course, if you were only a boss demanding a quicker pace, or a manager checking on his team’s performance.
To inspire true change you must also behave like a leader, being the first to deliver quality results, faster. No doubt your group will follow through.