As long as it's not detrimental to the quality of the final product, I believe all freelancers should be able to deliver projects as fast as they like. Having said that, I find myself earning less money by delivering top-quality projects sooner rather than later as other less-experienced freelancers earn more by taking longer to complete their jobs.
There are many industries that charge extra for meeting clients' requests sooner, so what would be the best way to apply this to freelancing apart from simply increasing the hourly rate?