It’s a function of ZFS itself. Data that is to be written to the drives is first written to RAM, then transferred to the drives. One of the benefits of this is that if you are moving a file that is smaller than the available RAM, your transfer won’t appear to be limited to the write speed of the drives.
Assuming my setup is typical, the dryer is on a 240V circuit. The washing machine is on a 15A 120V circuit.