I have a few ZFS ZPOOLs configured:

NAME      SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
backup   1.81T  1.41M  1.81T        -         -     0%     0%  1.00x    ONLINE  -
storage  21.8T  7.47T  14.3T        -         -     0%    34%  1.00x    ONLINE  -
tank     87.3T  19.2T  68.1T        -         -     0%    22%  1.00x    ONLINE  -

...but when I enter a directory ("cd /tank") it takes about two seconds or longer. When I perform a directory listing it's the same result. However, after it "warms up" then everything is instant.

What might be causing this?

I'm using a Norco RPC-4220 enclosure with LSI/Avago 9211-8i cards in IT mode. The drives are WD Ultrastar DC 550s (on one array) and WD Reds (different array) and are connected to the backplane using SATA and then cards are connected using SAS (I think?).

I don't think the drives themselves are spinning down because they don't seem to support it?

hdparm -B /dev/disk/by-id/ata-WDC_WD40EFZX-68AWUN0_WD-WX12DB0LN94R

/dev/disk/by-id/ata-WDC_WD40EFZX-68AWUN0_WD-WX12DB0LN94R:

APM_level = not supported

The only thing I can think of is that my server's RAM is 70% full which might be causing some problems?