From github.com/stephan-hof:

Hi,

I'm opening wiredtiger with the described setting above. I'm aware of the consequences: In a power-off scenario my database is corrupted or not usable anymore, but in my case this is acceptable (strong replication).

Still I can see on each checkpoint interval (60 seconds):

heavy disk usage
decreased throughput in my application

I looked a bit through the wiredtiger code and found out that this line of code is the culprit:
https://github.com/wiredtiger/wiredtiger/blob/2.7.0/src/btree/bt_sync.c#L253

I would consider this as a bug, this setting says "Please do not sync", but still the code does.

Another thing, the comment there says

Start a flush to stable storage, but don't wait for it

Which is not entirely true. If you look at the man-page it says that the call can block if there is already much disk IO happening.
http://man7.org/linux/man-pages/man2/sync_file_range.2.html