QFileDialog: generates unnecessary network traffic when browsing a mounted network drive

When using QFileDialog to browse a mounted network drive it seems to generate network traffic that possibly could be avoided.

When browsing a mounted Samba drive with QFileDialog on Mac:

1) The file dialog takes about 10 seconds to display the Samba folder, which
contains 20-30 items. (For comparison, the standard MacOS file requester in
the standarddialogs example app in Qt4.2.3 takes about 300 milliseconds to
display the same folder). During this 10 second period, the smbd process on the
erver was using 60-80% of the CPU consistently.

2) While the file dialog is open, the smbd process on the Samba server
consistently uses CPU cycles. Initially it consistently uses about 6% of the
server's CPU, but as I navigate around through various (small, 20-30 items)
directories in the Samba share, the amount of CPU cycles rises consistently by
several percent each time I change the current directory (e.g. by
double-clicking on a folder, or clicking on the "back" or "parent directory"
icons in the file requester). It seems that QFileDialog is caching
previously visited directories (they do load much more quickly, in about
100ms, the second time I visit them) but doing so in such a way that there is
a constant load on the smbd server that is proportional to the total number
of items in the cache.

3) The file requester is much more responsive after the
directory has finished loading. In particular, moving the selection or
scrolling the view up and down no longer result in increased network traffic,
and no longer lag while waiting for network communication to finish.