Too many to guess.
See here to start maybe.
http://www.accs.com/p_and_p/ZFS/ZFS.PDF
We need to know zfs implementation to start. Where did you get it and your distro.
Best would be management of how much you need to have data as in raid-z or other. Number of disks.
Types of data or size of data needed.
Don't forget to look at compression on disk. While it can help some if not all files, it can't improve many media type files.
Your hardware to some lesser degree but how system uses media may help. Iscsi or similar attached may have a great effect. ZFS sucks up memory.
ZFS has many variables for your use. As in most things, you sacrifice speed for security.
ZFS tends to be like BTRFS, use most current stable.
Many web pages offer clues for you to try. Test your config.
