Set the default value of scrub delay to 4 and resilver delay to 2
With the addition of the sequential scrub/resilver code, vfs.zfs.scrub_delay and vfs.zfs.resilver_delay default to zero. Normally, these default to a scrub delay of 4 and a resilver delay of 2. These delays effectively limit the maximum IOPS that can be used by a scrub or resilver, leaving some capacity available for normal use of the system.
The standard defaults of 4 for scrub and 2 for resilver should be restored.
For more information: https://www.freebsd.org/doc/handbook/zfs-advanced.html#zfs-advanced-tuning