I've noticed that with Mountain Lion (running on a mid-2011 iMac w/SSD) that once a download has started in Safari, or iTunes or Finder (AFP), etc. that any subsequent network activity is severely bottlenecked. It's as if the first download is filling my entire pipe and subsequent requests are given a lower (or NO) priority.
This didn't happen under Lion (or any previous OS X cat) and nothing has changed with my broadband Internet connectivity/bandwidth. Is there something I can do to make ML distribute the network bandwidth more 'cooperatively' so that when I'm waiting for some download I can at least browse the web or check my email?!?
This isn't localized to any one app (iTunes, Safari, etc.) it seems to be endemic to the entire system...and only started under Mountain Lion.