Are you sure it's not a personality clash?
Are the jobs being asked of this employee part of his job spec.
Is he doing any other tasks that you are not aware of that the others are not doing. This may be taking his time.
Do you set a completion date for tasks to be finished or do you leave it open to interpretation?
One of the functions of management is motivating employees. Have you looked at how you do this.
Is the employee under stress or are there any factors outside of work that could be putting the employee under pressure.
Are you discriminating against this employee.
Do you perhaps see this employee as a threat to your position.
Are you being fair and honest in your assessmnet of this employee.
Are the tasks you assign specific, measurable, achievable, realistic and with a time frame around completion.
Does the employee need training. If yes, what training courses have you identified to help this employee. Time management, Assertiveness course?
When asking employees to do a job, Are you giving a clear an unambigous brief. If the brief is not clear then that is your fault.
There may be dozens of factors. Just make sure you have illiminated all of them before you go down the warnings/HR route. By all means seek guidance from HR but don't go in gung ho.I have seen this backfire on supervisors and managers in the past. Particularly if you work in a unionised company. In a unionised company it is easier to get rid of the non unionised manager than the unionised employee!