Expensive queries can impact overall performance. By adding an index to a column being used in a query, it can significantly improve overall query times. Here's a guide for profiling the database.
First, connect to the database using the mongo shell. If you're not already in the target database, then run:
use database_name;
Next, turn on the profiler, specifying the number of milliseconds required for a query to be logged:
db.setProfilingLevel(1, 300); // logs anything slower than 300ms
Finally, to display the last 10 logged queries, do:
db.system.profile.find().limit(10).sort( { ts : -1 } ).pretty()
When you're done, be sure to turn off the profiler:
db.setProfilingLevel(0);