Jens Segers recently create a pull request to Laravel to make creating custom mail drivers easier. During the process he decided to do simple performance test and see how much a difference it is between using just SMTP vs an API:
At first sight, it looks like the SMTP driver is faster than the API driver for both Postmark and Mandrill. So if you are not using any specific API features, it looks like you can just stick with the default SMTP driver.
