You are approaching the type of speeds where latency will DRASTICALLY affect your download speeds. Because of the latencies, it takes longer to acknowledge TCP packets, and thus, it slows your connections down. It does unfortunately start to affect download speeds when TCP-ACK packets take long to arrive. The STREAMing extensions of NNTP (which speeds up Usenet drastically) is only supported between server to server peering, not between server to client connections.
That being said, even software, PCs, network devices, etc. for users in the EU, using EU based servers, needs to be severely tweaked to go faster. It's the way that the protocol works that essentially slows it down. Even for EU users with sub 10ms latency to the NNTP servers, getting more that ~100Mbps takes a lot of effort.
What you download (new / cached content vs. old content) will affect your speeds. New / Cached content is frequently put on fast SSD storage, whilst older content is placed on cheap, old SATA-3 drives. The one can obviously handle a lot more random IO than the other, and thus speeds can either increase or decrease based on where the content is coming from.
Frequently, when articles aren't found, providers will delay the not found error in order to stop the client from hammering the server. Again, there's a delay and it will slow your overall downloads down (even more so when there's a lot of articles not found).
Lastly, not directed at you of course. I take offence to the comment of "dodgy" networks. We are fairly small in the NNTP industry, and I can assure you there's nothing dodgy about the networks or infrastructure. We being a small player, sit with multiple 10G circuits, multiple racks full of servers (literally), and multiple public/private peering arrangements. At any given time, we push about 14Gbps+ in traffic... There's nothing dodgy, and nothing small, about usenet. Literally having racks and racks full of nothing but hard drives, isn't cheap...
If you can set your SAB logs to full debug (no confidential info is logged) and send me the log after you downloaded something I would more than likely be able to tell you what and why things are slow (if it's a SAB or server issue, at least). I would also suggest you try a different client (say Grabit) to just test and compare speeds, and if you do use SAB, that you upgrade to the latest versions. SAB has done enormous amounts of work in recent versions to optimize and speed up the decoders - there are drastic improvements in recent versions.