Rsync slow

I am rsyncing from one NAS on my LAN to another.

• If I go from NAS1 to NAS2(EDA500), the transfer goes at aout 25MB/sec. That's cool.
• If I go from NAS1 to NAS2, it transfers about that speed, but between files it takes literally forever to move to the next file, and the transfer speed drops to 100KB/sec.
• I've made sure I've done a defrag and a balance. I should also note that I get the same problem if I rsync between NAS2 to NAS2(EDA500), but not from NAS2(EDA500) to NAS2(OTHER EDA500).

This is what I see on NAS2 during this pause:

Cpu(s): 0.5 us, 3.8 sy, 0.0 ni, 14.1 id, 81.1 wa, 0.0 hi, 0.4 si, 0.0 st
KiB Mem: 2030856 total, 1888888 used, 141968 free, 4484 buffers
KiB Swap: 1569788 total, 140760 used, 1429028 free. 939876 cached Mem

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2037 root 20 0 0 0 0 D 3.6 0.0 68:52.48 md125_raid5
2056 root 20 0 0 0 0 D 3.0 0.0 68:16.66 md124_raid5
2176 root 20 0 0 0 0 S 2.6 0.0 8:18.35 md118_raid5
5481 root 19 -1 1772156 15716 5008 S 1.3 0.8 36:39.50 readynasd
23662 root 20 0 315420 17712 2916 S 1.3 0.9 58:40.95 sshd
4469 root 20 0 0 0 0 S 1.0 0.0 4:46.09 btrfs-tran+
1723 root 0 -20 0 0 0 S 0.7 0.0 22:21.20 kworker/1:+

And this is what I see when the transfer is working OK:

Tasks: 342 total, 3 running, 339 sleeping, 0 stopped, 0 zombie
%Cpu(s): 17.0 us, 10.4 sy, 0.0 ni, 28.6 id, 39.3 wa, 0.0 hi, 4.7 si, 0.0 st
KiB Mem: 2030856 total, 1856096 used, 174760 free, 9188 buffers
KiB Swap: 1569788 total, 140756 used, 1429032 free. 969308 cached Mem

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
23662 root 20 0 315420 17712 2916 R 79.9 0.9 58:52.42 sshd
23693 root 20 0 124520 2352 0 D 11.9 0.1 8:30.14 rsync
2037 root 20 0 0 0 0 D 9.6 0.0 68:53.99 md125_raid5
2594 root 20 0 0 0 0 S 3.3 0.0 0:00.43 kworker/u8+
1803 root 20 0 0 0 0 S 2.6 0.0 0:04.23 kworker/u8+
18347 root 20 0 0 0 0 S 2.6 0.0 0:47.71 kworker/u8+