I noticed a very poor performance of http downloads in my application. During my investigations I found that disabling I/O buffering fixes this problem.
Internally Qt uses fopen() for buffered I/O and open() for unbuffered (see openModeToFopenMode() in qfsfileengine_unix.cpp). BlackBerry/QNX documentation  suggests to prevent usage of fopen() for file I/O because of FS buffering.
I have attached a small test application for BlackBerry 10. Everyone can see a difference between buffered and unbuffered I/O by a simple change of QFile::open() parameters in downloadmanager.cpp:27.