At http://docs.mongodb.org/manual/administration/production-notes/#kernel-and-file-systems

Suggest after "As such, you should use the Ext4 and XFS file systems:" and before the following bullet points add something like:

We recommend using XFS if possible because we find it in general performs better with MongoDB. This is particularly important when using the WiredTiger storage engine, in which case we strongly recommend using XFS to avoid performance issues that we have traced to using EXT4 with WiredTiger.