Presently mmap'ed file flush happens in sequence for multiple files. This results in long flush times on Azure / Windows platform where the OS is not able to do concurrent flushes of the contents of the file. See
SERVER-12401 for more details.
The issue is especially critical if there are lot of random updates that dirty large parts of the mmap'ed region. At some point (Q3'14?) Microsoft will release the fix for the Windows Cache Manager, and we will be able to flush file regions in parallel. It will provide better performance than
SERVER-12733 on average.