Might I suggest offering AutoPar to be run at the end of a download Queue or even manually?
This only works for you. Won't work for people who have TB or 2 in the download list or who never let their download lists empty.
viewtopic.php?f=43&t=30069Is a way to increase the buffering Newsbin does when the disks slow down. In this post, the user says it solved his "slowing download while unrar" issue. There's another method which lets Newsbin keep the whole file in memory before writing it out. I'd try the packet buffers first. It's more reliable.
You could also set Newsbin to unrar to a different drive, which should reduce the load on the current drive. I can download and unrar while maintaining 200+ Mbps download speed without using the packet buffer tweek so, you should be able to too. This depends on your download disks of course and CPU power.
I'm not keen on deferring all unrar processing till the end. I'm not saying no but, it's not really how I see Newsbin working.