There's a fairly simple reason for that which is that ZFS (and btrfs to some extent) are almost literally "ACID" databases. They do alot of the same double-writes and other safe behaviour the database is also doing. Those have a penalty and you're doubled up.
There are various guides around for tuning ZFS and database servers to try reduce that duplication, for example you can disable the InnoDB double write buffer because ZFS guarantees you don't need it. You also need to tune recordsize to match the database page size so that you don't accidentally create large multi page blocks.
There are various guides around for tuning ZFS and database servers to try reduce that duplication, for example you can disable the InnoDB double write buffer because ZFS guarantees you don't need it. You also need to tune recordsize to match the database page size so that you don't accidentally create large multi page blocks.