I am experiencing the same kind of issue with my Synology 1512+. So far, I have read that many encounters slower download speeds with SABnzbd and I am kinda frustrated because I really do love my SAB!
As for my esperience: I recently switched over to FO and had my SAB install previously on a Mac Mini and was getting 136Mbit/s.
More recently, I bought a Synology 1512+ and had SABnzbd installed through the Synocommunity package. SAB itself is behaving perfectly well but I'm only getting, at max: 56Mbit/s. So one would argue that I should not complain with that kind of speed. Fact is that when you know you have more, you always want more
And more seriously, I get a drastric 40% loss in download speed performance! Kinda frustrating.
At first, I thought it was my NAS but I ssh to my NAS, did a wget on some ftp website and did reach my 136Mbit/s.
Now, I read some article and found out about the max # of connection, the cache articles: Reduced from 20 to 6 until I started to see a decrease in my 56Mbit/s download speed. Then started playing around with the cache setting from null to 100M. Nothing has either improved or worsen...
I also want to add that at no moments, my NAS gets overloaded. Overall memory usage is at 30% and CPU usage never gets passed 50% at max.
Has anybody got any idea? Thanks for your inputs!