I've been using mongo on 39M+ records (tracking financial tick data) across 73 assets (collections) and my queries take anywhere between 2-3mins depending on complexity.

You can always run db.currentOp() in the mongo shell to see what process is taking forever as well.

Let me clarify why though, there methods of optimizing a query by adding another field, but since I have to traverse my records with the sort() cursor my queries take that long.