I would recommend checking out Kadira for optimizing your app. It's pretty amazing the kind of information you get out of it, and Kadira Academy teaches you a lot about Meteor's internals.

Also, if your app is now "suddenly slow", I'm guessing it's been around for a while. If you don't have the proper indexes on MongoDB it's going to get slower as it loads up with more data. I've experienced this first-hand, migrating a Rails app from Postgres to Meteor/MongoDB. We have millions of records, and one missed or incorrect index can slow the whole app down, sometimes unbearably.