Speed test sites are usually not much use in determining ISP throttling because the download is often too small to trigger the throttle. (better yet, download a big file while watching a bandwidth graph)
It appears it's not a problem that originates at Astraweb's end - they have super-fast connections and don't throttle - and your internet connection (between the Astraweb server and you) was previously capable of much higher speeds, so there's no reason for the slowdown -- unless your ISP is (intentionally) throttling you.
Maybe you found yourself in Hog Heaven and downloaded so much your first few days of having usenet that you tripped the ISP's "bandwidth hog" switch?
It helps to use a standalone bandwidth meter to check things out (I use NetMeter -
www.metal-machine.de/readerror/ ) instead of relying on "speed test" sites or watching numbers.
But even if your ISP is throttling you, there might still be ways around it.
There are many ways a ISP can throttle you: by (total) internet connection, by connection stream, by port, by packet-signature, by destination IP address, etc. Ok, so the goal is to thwart each of the other possible throttling methods. You've already tried changing ports and increasing connection streams and using SSL on port 443. (So maybe try downloading from multiple IP addresses at once?)
Some things to try:
Download a file from Astraweb using just one connection (watch the speed)
with usenet downloads running, then download a file from some HTTP or FTP site. Does your total bandwidth increase?
Have you tried downloading from more than one of Astraweb's 4 servers at a time? (set each file to download from a different server) or 2 NSPs at once?
You might also try a trial usenet account from another company or a VPN/proxy.
If, in the end, none of this works to increase your speed, then it's possible that your entire ISP connection is being throttled. In which case there's nothing you can do.