I also posted 2 specific examples - EasyNews and Fileplanet - so it seems a good place to start might be Fileplanet - as two of us have mentioned that one as being slow with specific proxies and speedier or full speed with other specific proxies, but to be honest I find I can choose almost any USA site and find it downloads files at half the speed compared to full expected 3 meg speed with specified proxies of Colchester or Hersham or Swansea.
Try Fileplanet with Poplar Proxies - then try Fileplanet with Colchester proxies for example.
Easynews have a trace here that includes a speed measurement.
http://www.easynews.com/slow/trace.phtml
cheers
Ian