SABnzbd is a nice tool. I just replaced my previous nzbget with it, due to its better handling of the obfuscated names in usenet groups. However, on an Atom CPU, the max download speeds did not go over ~5MB/s on a 100Mb/s link. This is rather sad, because nzbget did get the whole ~11MB/s speeds.
The source of this slowness is the handling of the SSL within python (or SABnzbd, to be exact) which cannot exceed a single core. A workaround for this problem was using stunnel with the SABnzbd, as described here. Because stunnel uses multiple cores, handling of the connections can exceed the rather weak CPU limit into other cores. Following this procedure, I was able to reach 11-12MB/s speeds.
Notice a little correction: The bottom string: connect, has to have the equal sign, like this:
connect = news.giganews.com:563