You have not mentioned how long it has been since the invitations were sent out. Without knowing how much time has elapsed, it is difficult to understand whether the response time was “questionable” as you have suggested. It is possible that since the other reviewers have been very prompt, the time taken by the unresponsive reviewer is appearing too long to you in comparison. In fact, I think the other two reviewers have been unusually quick. I have not heard of a review being completed in 5 days: reviewers usually take a few weeks or even months complete a review. If it has been just a few weeks since the review invitations were sent out, I would not consider it a very long time.
Generally, journals have a policy about the number of days reviewers are given to respond to a review invitation. Once the stipulated time frame is over, the Editor either sends a reminder or starts looking for another reviewer. This is part of the Editor’s responsibility, and I’m sure he or she will invite another reviewer once the response time has elapsed. If it has been just a few weeks, I don’t think you need to write to the Editor about this.
However, if the status continues to remain the same for more than 2 months, you can write to the Editor and inquire about the progress of the peer review. You can also offer to suggest reviewers if required.