Explanation/information sought: Windows write I/O performance with "fsync" (FlushFileBuffers)
You're seeing an exponential decrease in speed on the sync runs because these aren't purely sequential workloads as you believe. Since you're starting with a new file each time, your writes are growing the file and the metadata needs to be updated in the filesystem. That requires multiple seeks, and as the file grows the seeks from the end of the file to the metadata take longer and longer. I also posted this on your other question by mistake, see the full answer there: https://stackoverflow.com/a/18429712/894520
Try turning off disk caching and repost?
Otherwise these metrics are nonsense (Fsync and write through may not actually hit disk). Windows by default enables disk caching and controller caching..
Greg