When performing a basic mongodump on our database, with settings seen below, on high performance hardware, we're seeing very long delays between writes, and nearly zero resources on the system being utilized, but significantly increased response time to even the most simple of commands.

CPU utilization remains the same as other systems. iostat shows nearly no activity, similar to other systems. Network load remains the same.

The time it takes across NFS is just as long, and more easily shows the prolonged gaps in activity. You will see random peaks of full bandwidth usage lasting a few seconds, followed by several seconds, sometimes minutes of zero activity.