Recently I've been having issues with my Synology DS1512+. It had been slower than usual and I had random issues connecting to it. The web interface was dog slow too. I just associated the issues with my volume being 94-99% full. I'm running the latest version of DSM as I write this (5.1-5021 Update 2) and I have 4GB of ram installed.
I was flipping through the settings looking for something else and ran across a heading "Memory Compression" and under it was only one option "Enable Memory Compression to improve system responsiveness" which sounds like a no brainer when you read it like that. That's like asking me if I want to go slow or fast... of course I want to go fast. What this option seems to do is write "lesser used" ram to the page file. I'm not sure what it's using to determine this but I think they might want to tweak it. After unchecking and restarting it's back to its peppy self again. Right back to topping out my PC's nic, no more strange issues connecting, and the web gui was back to normal. My memory utilization is sitting right at 4% so I don't think I need to worry about running out of ram on this thing any time soon.
TL;DR Turn off "Memory Compression" on your Synology NAS unless you really need it.