I'd say that the faster way to do it is with interruptions, in this case with attachInterrupt(). On the other hand, you can get the information from the Arduino sketch and use it in the Node.js script. The following document describes how to share information between environments, you might find it interesting: IoT - Data sharing between programming environments on Intel® Edison | Intel® Developer Zone
I read the article some time ago (definitely thinking outside the box ) since IPC can be done in many linux-oriented ways) and I discarded it since I believed that MRAA will be the answer to everything. Now, I might be tempted to try something similar. I think that a somewhat "model-view-controller" approach could work here to get maximum performance from this little project. Meaning I could use Arduino to fully control the DC motor and use NodeJS for the web server.
I'm just curios if for this rather simple IPC use case there are more linux oriented approaches, because technically I would still rely on the mraa interrupt thread to notify node when the sketch has data to send.
There are attempts to use shared memory, for which there is some support, but I haven't seen any simple examples. Also, I don't know if arduino sketches work well with Unix sockets (as an alternative). From this perspective, your suggestion is easier to implement and test.