At least one user has noted that the concurrent uploading of multiple files saturates his Internet connection and slows down his overall Internet access unacceptably when making a large batch upload.
In addition to the site-level configuration to define the number of maximum simultaneous upload requests, it may be useful to add a user preference for concurrent uploading to empower users of slower Internet connection to turn off this future.
Version: unspecified
Severity: enhancement
How much benefit do we get from concurrent uploads?
We might consider simply always uploading sequentially, or at most two simultaneous uploads so we get "full speed" on the other channel during the connection setup phase.
This is currently a server-side config setting, Brion, so any server can decide to only allow one concurrent upload (sequential) or two (like you suggested) or five hundred (potentially an issue).
Adding an extra user preference and using the lowest between the server config and the user preference shouldn't be altogether too difficult.
I don't think anyone's really done a formal evaluation of the performance benefits - it would be nice to at least do some more crowdsourced before/after comparison for batches to see what the benefits of the feature are. That, too, would be easier if it was a user preference. ;-)
FWIW, I got someone in IRC saying that this user preference would be nice for folks with really slow connections....
http://bots.wmflabs.org/~petrb/logs/%23mediawiki/20120826.txt
Search for "Juandev" and you'll get the start of the conversation.
There's something wonky about how the preference gets added, which likely caused bug 48090. The preference itself is actually not changeable (try changing it to a different value), and mw.user.options.get('upwiz_maxsimultaneous') returns null.