I don't see the value in this service since markup conversion is built in to so many free libraries and services. What is the use case for this and who needs such as service?
Kramdown does it in pure ruby, and stupid fast as far as I can tell, and that's single threaded with the GIL.
But you'll have to maintain it, scale it, etc..that's the beauty of cloud APIs. You buy, and leave the boring part to others.