I'm having some slowdown issues as well. They seem to have started when I installed a 6.62 beta version over 6.61. I did upgrade to 6.62 when it was released and things were still slow. I tried going back to 6.61, but that didn't solve it. (It's of course very possible that it was a coincidence that the slowdown started when I upgraded...)
I'm also seeing high cache numbers. 166/200 (16) if that has anything to do with this.
I was seeing normal speeds when I first started NBP, and then they would creep down to very little (1 or 2 Mbps shown in NBP). I also have an applet that shows traffic in/out on my network interface, and in that I was seeing it jump up, then drop, and jump up, etc. It seemed very chaotic, unlike it used to be when I saw steady amounts of incoming traffic.
I had also checked with my NSP and they reported no problems with their servers or network. Internet speed tests showed that, at least for HTTP, my ISP's network was running ok. I tried power cycling my router, and also switching to a wired connection from the router instead of wireless. I turn my system off when I'm not using it, so it probably doesn't have much in the way of apps with memory leaks slowly consuming memory. (It doesn't matter what other apps are running or not running either. The slowdown is the same.)
I've played with the performance settings in NBP, tried d/ling files of varying sizes, and from different newsgroups. I'm using Linux Mint 17.2 64-bit with 3.16.0-38-generic #52~14.04.1-Ubuntu SMP kernel. System RAM is 16GB and it looks like this with NBP and a few other programs running:
total used free shared buffers cached
Mem: 16432924 15872212 560712 90784 254732 10073080
-/+ buffers/cache: 5544400 10888524
Swap: 16740348 13888 16726460
I use Crossover 14.14 with NBP in its own bottle, an XP one. disks are spinning, and there is plenty of space.
The last few days things got even worse, and I can't add new items to the download queue, plus a couple of Download Headers jobs I started aren't completing, nor are downloads. The max traffic I see from NBP is 30 B/sec down. I've even tried deleting the header jobs and it doesn't help. In addition, if I shut down and restart NBP, the header jobs reappear.
I also have a Win 7 VM (32-bit) that I play with from time to time. I installed Agent 8 on this and tried it with my NSP. For NBP, I had arrived at 15 connections (SSL) in the past as seeming to do well. I tried that with Agent and saw the same kind of behavior, with chaotic actual download speeds, and overall nowhere near the speeds I should be seeing. I tried increasing the number of connections to almost 30 and didn't see any increase in speed. Then I dropped the number of connections way down to 6 and suddenly I was seeing steady download speeds of the speed I was used to in the past.
I think that narrows things down to an issue with NBP or Crossover, or an interaction between one or both of these and Linux Mint. After trying Agent, I dropped the number of connections in NBP down to 6 and then 5 and it unfortunately didn't help.
Any ideas on where I might try looking next?