Hi!
I use mjml and python/django by command mjml and pipe-line for pass template and get result.
It is very simple but very slow -- 0.8 - 2 seconds.
https://github.com/liminspace/django-mjml/blob/master/mjml/tools.py#L9

Then I try make tcp-server for keep mjml instance in process and I get better speed -- 0.1-0.2 seconds.
https://github.com/liminspace/django-mjml/blob/master/mjml/node/tcpserver.js

I am not good in node.js, so I propose you make mjml-api-server that other non-node.js services can use it as fast as possible.