With the introduction of the 'Storage System Optimization' in 18.104.22.168. It is recommended to use that instead of making manual changes as per the below. These instructions are intended for systems on versions prior.
There are two kernel parameters that can adjust the speed of a resilver - 'zfs_resilver_delay' and 'zfs_resilver_min_time_ms'. Changing these will increase, or decrease the speed.
To increase speed of a resilver:
You can speed up the resilver by doing the following as root:
echo 1 > /sys/module/zfs/parameters/zfs_resilver_delay
echo 5000 > /sys/module/zfs/parameters/zfs_resilver_min_time_ms
If you find that the resilver is still going too slow, you may change the "1" in 'zfs_resilver_delay' to "0".
WARNING - Changing this value is only recommended if a resilver needs to take priority over performance, as performance is likely to be affected after making the below change.
echo 0 > /sys/module/zfs/parameters/zfs_resilver_delay
When the resilver is done you can reset it back to the defaults by doing the following:
echo 2 > /sys/module/zfs/parameters/zfs_resilver_delay
echo 3000 > /sys/module/zfs/parameters/zfs_resilver_min_time_ms