Comment 3 for bug 514725

HellaNZB indeed has a poor bandwidth throttling algorithm. It just seems to download at maximum speed for a while and stops downloading after that so that the average speed will match the specified limit. According to the HellaNZB developer (http://hellanzb.com/trac/hellanzb/ticket/45) the problem lies in the Twisted code.

Regarding SABnzbd: It's definitely much better at throttling the download speed.