Wondering how to handle a slow employee!

I have a writer for producing sales copy, web content and other contents. He is very good writer but he always miss the deadline. He writes very smart with premium touch. I usually give him 2-3 days for about 2 contents, though I expect he at least deliver in 4-5 days, and I have no problem with it. But he always takes 7-10 days to deliver the contents (sometimes even more). I don’t fire him just because he writes very good content. I want to keep him at hand so as to I can use him when I need good contents. So I deliberately increased his rate and told him to be quick – my bad, still no improvement! Then I pretended that I’m reluctant to give him work anymore. So he promised he won’t delay anymore. Believe me this time he took some more days :D He always makes promise but never keep it. He shows many excuses I believe they are not true at all. At times I give him bonus to motivate him. But nothing works for him. I have been working with him for more than 2 years and he was comparatively quick at the beginning days. Have you ever encountered similar issues? What would you do if you had faced the problem?