After upgrading a replica set to 3.2.9, I am seeing:

• increased CPU usage on the two hosts that have SSD's
• increased CPU and IOWAIT on the host that has regular HDDs (in software RAID1, if it makes any difference)

The workload hasn't changed before/after the upgrade (about 40-60 inserts / 1-2 queries / 0 updates / 0 deletes – per second)

The configuration also hasn't changed, I've been running WiredTiger before as well.

The storage configuration is configured as follows:

storage:
  dbPath: /var/lib/mongodb
  directoryPerDB: true
  journal:
    enabled: true
  engine: "wiredTiger"
  wiredTiger:
        engineConfig: 
            journalCompressor: zlib
        collectionConfig: 
            blockCompressor: zlib 
        indexConfig:
            prefixCompression: true

I've attached diagnostic.data as requested and some munin charts that show the difference before/after.

Seemingly related issues:

• SERVER-25883
• SERVER-25700

Let me know if there's anything else I can do to help.