- what is the oplog size in db.oplog.rs.stats().
It should use 5% of your total disk space.
If you believe it is too large, you can try reducing it on command
line (but requires special care once created).
The bottom line is to be able to have a server down for a few days and
still be able to catch up.
- based on the oplog size, it will reported as is in virtual size, but
it does not mean it takes that much ram.
The slow query you show is very slow indeed, is it always that bad?
How often does it happen?
If secondaries are close in time, they should only hit recent oplog
pages which should be in RAM, hence fast.