Windows, clean profile, in system (IE) the proxy is configured as none. We still are doing PAC resolution for every request that adds one more main-thread loop. I don't exactly recall the reason for this, but I believe we've discussed this yet in MV that there is no need to do this. Patrick?

I think we need to do this on windows because we don't know that the system lookup is going to return NONE all the time, and that function needs to be executed off the main thread for that platform (it has a nasty habit of blocking on WPAD queries at times that are opaque to firefox). The discussion you might be remembering was about android.. which actually could synchronously return NONE in its default config, but we always force a trip through the event loop. We still do that - I've got a patch that I haven't posted yet (its just tied up in a patch queue of other stuff) but I intend to submit it as part of the current nightly cycle.