In the case of video media, the folder, fanart, nzb, sfv, and nfo files would all stay on that local hard drive,
Newsbin moves anything mentioned in the PAR file to the unrar destination too.
Seems like you have to eat the slow network drive on the front end or the back. You either take the hit during download or take the hit in the subsequent file copy. The only way you'd get full copy speed is if you didn't do it while Newsbin was also downloading. There's only so much disk bandwidth available. I imagine the process of copying the files off the local disk to the net drive would slow download too.
In the performance options:
1 - You could try turning off buffer mode and see if it makes any difference.
2 - You could tell Newsbin to stop writing the download to disk while it's repairing/unraring.
3- You could increase the internal chunk buffer size so, it potentially keeps much more of the file in memory. That's the thing in the cache line. I have mine set to 2000 right now. That gives Newsbin up to 1G of memory to buffer the downloads into.
I'd restart Newsbin to be sure the settings took.
In 6.50, you'll be able to decouple the download chunks from the assembly. So, you could download chunks to the internal drive and have them assemble on the network drive. It's not clear how much of the slowdown is because of PAR processing. The file has to be scanned to find all the par blocks. That might be what's slow about the network drive..