We have not benchmarked Drone.io, I will do one over the weekend and let you know. In the meantime I would love to hear your experience with Shippable.

We have been running on Docker since Oct 2013 and we have fine tuned our based on that experience which helps to make it fast.

Yup, we do save containers into our local Docker repo which gives you added speed as a result of caching. Since we maintain state of all your containers for every change set, we can go back to any version and hence it allows to remove layers and reset minions (containers)

We will be announcing some key updates soon that will make it completely YML driven. Please stay tuned..Hope this helps