I have made my peace with the fact that NTFS(-3g) on Linux will be slower than NTFS performance on Windows. I can write to my external NTFS-formatted USB 3.0 HDD at about 100+ MB/s on Windows while I have to settle for 30 MB/s (give or take) on my Debian (Wheezy) box.
That's not the problem, however. I found (empirically) that if I want to copy, say, 20 files from my box to the HDD, the copying starts off at the "normal" 30 MB/s but gradually slows down to a miserly 4 MB/s!! Whereas, if I do the copy, say, 5 files at a time (serially) the copying speed remains at 30 MB/s for all four copy processes. This is not specific to Debian, by the way. I've observed similar behavior on Fedora and Ubuntu.
My question is, is this behavior normal or should I be concerned? If something is wrong, how should I go about debugging it/fixing it?