I think it has to do with Meteor real-time pub/sub system which tails on the Mongo Oplog, and I’m not clear on when a fine tuned app that uses Meteor’s pub/sub start showing performance issues and how severe are those. I assume if you don’t use pub/sub then it’ll have the same scaling behavior as your typical node.js app.
You can follow the Redis-oplog thread for more info on how to solve scaling the pub/sub system.
I hope someone with firsthand experience on scaling Meteor pub/subs system can provide more insight, these are just my assumptions.