your system is reporting the cpu isn't very busy, so if all pages are sluggish it's probably from the disk. What is your PG shared buffer size? May want to increase that. Too bad you're not running 7.2; if you were, you could check the PG cache hit rate directly instead of guessing. If you could show iostat from a period of sluggishness, rather than the global averages since reboot, that would be more informative. From your average it doesn't look like you'd gain much from moving things to different disks, but that could be misleading.
Another thing you can check is how many threads you are giving nsd. I know from experience that setting this too high can cause slowdown for no apparent reason.
