For those of you who are lucky enough to have a really high bandwidth (like 20+ Mbps), SABnzbd requires careful setup.
For lower speeds the configuration of SABnzbd is not very critical.
For higher speeds SABnzbd sometimes needs to throttle the actual download speed, due to high disk activity.
You can often avoid this throttling by setting a large memory cache in the configuration screen.
A typical RAR-segment size in a download is 50MB. It helps if SABnzbd can keep such a segment completely in memory. Therefore you need to set a cache limit of about 60M to70M in the cache parameter.
This only helps when you have enough memory to prevent the Operating System from swapping memory to disk.
Of course when downloading is consistently faster than processing, nothing helps and SABnzbd will have to limit the download speed anyway.