Comment by treeder

Unfortunately, Docker has a fair bit of overhead, more on that here: https://medium.com/travis-on-docker/the-overhead-of-docker-r... . That said, the overhead is probably not a big deal in a lot of cases.

And for the those cases, where it is a big deal, the next two major milestones on the roadmap aim to work around that issue, namely hot containers and streaming input. https://github.com/iron-io/functions#roadmap . Once that's in place, it should perform similar to any API.