I regularly get 350KB/sec from HTTP, and always get that kinda speed from FTP and NNTP. Obviously, FTP and NNTP aren't dependent on the proxies.
The occasions that i don't get full speed from HTTP, i'll assume the webserver can't spit it out that fast.
I haven't done extensive tests on the same file over and over again, at different times of the day, to ascertain if i regularly get the same speeds from the same servers.
it definitely looks like some proxies are faster than others, though. I don't know the areas you've spoken about, what's the difference in population density (and cable takeup?) in the areas?
Have you tried using a download manager (like at
www.intelligentdownloadmanager.com ) to see if you can speed things up on the slow proxies. that'll be able to tell you if the proxies can handle the traffic or not. Might be that some proxies are setup to limit the speed of an individual thread, but this can be circumvented by downloading a file over 4, 8, or even 16 threads.