[MovedGoalPosts]

When you are downloading each connection is only pulling one file at a time. That was fine for text, but I presume you are pulling somethings that will be made up of lots of files that SABnzbd is then reconstructing into the larger "document" once it has all been leeched. In that case you want a few connections, rather than just one so that each can overlap. See if there is a speed change with say 3 or 5 and perhaps experiment, just don't have too many otherwise they will all cancel each other out.

I assume you are on a wired rather than wireless connection to your router. Wireless could throw up all sorts of other issues with interference.