Meteor performance bottleneck - EDIT: poor oplog performance

Ugh, looks like oplog is slowing me down, according to Kadira. Latencies of 1200ms, 2200ms, etc. Same issue as this person:

Have I implemented something incorrectly? I thought oplog was supposed to improve performance.

Here’s an example of part of my publication that’s not performing so well:

Is this an issue with mLab? I’m on their lowest priced plan ($15/mo, shared). MongoDB Compass shows me that this query would’ve used the index: